Display device including transistor and manufacturing method thereof

ABSTRACT

An object is to provide a display device which operates stably with use of a transistor having stable electric characteristics. In manufacture of a display device using transistors in which an oxide semiconductor layer is used for a channel formation region, a gate electrode is further provided over at least a transistor which is applied to a driver circuit. In manufacture of a transistor in which an oxide semiconductor layer is used for a channel formation region, the oxide semiconductor layer is subjected to heat treatment so as to be dehydrated or dehydrogenated; thus, impurities such as moisture existing in an interface between the oxide semiconductor layer and the gate insulating layer provided below and in contact with the oxide semiconductor layer and an interface between the oxide semiconductor layer and a protective insulating layer provided on and in contact with the oxide semiconductor layer can be reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device including a circuitformed with a transistor, and a method for manufacturing the displaydevice.

2. Description of the Related Art

Various metal oxides exist and are used for a variety of applications.Indium oxide is a well-known material as a metal oxide and is used as alight-transmitting conductive material which is necessary for liquidcrystal displays and the like.

Some metal oxides have semiconductor characteristics. As metal oxideshaving semiconductor characteristics, for example, there are tungstenoxide, tin oxide, indium oxide, zinc oxide, and the like, and atransistor in which a channel formation region is formed using such ametal oxide having semiconductor characteristics has been proposed (forexample, see Patent Documents 1 to 4 and Non-Patent Document 1).

As metal oxides, multi-component oxides as well as single-componentoxides are known. For example, InGaO₃(ZnO)_(m) (m is a natural number)having a homologous series is known as a multi-component oxidesemiconductor including In, Ga, and Zn (see Non-Patent Documents 2 to4).

In addition, it has been confirmed that an oxide semiconductor layerincluding such an In—Ga—Zn-based oxide can be used as a channel layer ofa transistor (see Patent Document 5, and Non-Patent Documents 5 and 6).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No    S60-198861-   [Patent Document 2] Japanese Published Patent Application No.    H8-264794-   [Patent Document 3] Japanese Translation of PCT International    Application No. H11-505377-   [Patent Document 4] Japanese Published Patent Application No.    2000-150900-   [Patent Document 5] Japanese Published Patent Application No.    2004-103957-   [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G    Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M.    Wolf, “A ferroelectric transparent thin-film transistor”, Appl.    Phys. Lett., 17 Jun., 1996, Vol. 68, pp. 3650-3652-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,    “Syntheses and Single-Crystal Data of Homologous Compounds,    In₂O₃(ZnO)_(m) (m=3, 4, and 5), InGaO₃(ZnO)₃, and Ga₂O₃(ZnO)_(m)    (m=7, 8, 9, and 16) in the In₂O₃—ZnGa₂O₄—ZnO System”, J. Solid State    Chem., 1995, Vol. 116, pp. 170-178-   [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m)) (m: natural    number) and related compounds”, KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327-   [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.    Hirano, and H. Hosono, “Thin-film transistor fabricated in    single-crystalline transparent oxide semiconductor”, SCIENCE, 2003,    Vol. 300, pp. 1269-1272-   [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.    Hirano, and H. Hosono, “Room-temperature fabrication of transparent    flexible thin-film transistors using amorphous oxide    semiconductors”, NATURE, 2004, Vol. 432, pp. 488-492

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide atransistor having favorable electric characteristics and highreliability, and a manufacturing method thereof. Another object is toprovide a display device to which the transistor is applied and whichhas favorable display quality and high reliability.

One embodiment of the present invention is a display device in which atransistor including an oxide semiconductor layer is provided. An activematrix substrate of the display device includes a pixel portion and adriver circuit portion. A gate electrode is provided to overlap with aback channel portion of a transistor in at least the driver circuitportion. In manufacture of the transistor, the oxide semiconductor layeris subjected to heat treatment for dehydration or dehydrogenation. Afterthe heat treatment, a protective insulating layer is formed using aninsulating inorganic material containing oxygen so as to cover the oxidesemiconductor layer. Through the heat treatment, the carrierconcentration is changed.

A transistor having favorable electric characteristics can bemanufactured. In particular, a transistor whose threshold voltage is noteasily shifted even when it is used for a long term and which has highreliability can be manufactured. With use of such a transistor in atleast a driver circuit portion, the reliability of a display device canbe improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are diagrams each illustrating a transistor which is oneembodiment of the present invention.

FIGS. 2A to 2D are diagrams illustrating a transistor which is oneembodiment of the present invention.

FIG. 3 is a diagram illustrating an electric furnace that can be appliedto the present invention.

FIGS. 4A and 4B are diagrams each illustrating a transistor which is oneembodiment of the present invention.

FIGS. 5A to 5D are diagrams illustrating a transistor which is oneembodiment of the present invention.

FIGS. 6A and 6B are diagrams each illustrating a transistor which is oneembodiment of the present invention.

FIGS. 7A to 7D are diagrams illustrating a transistor which is oneembodiment of the present invention.

FIGS. 8A and 8B are diagrams each illustrating a transistor which is oneembodiment of the present invention.

FIGS. 9A to 9D are diagrams illustrating a transistor which is oneembodiment of the present invention.

FIGS. 10A and 10B are diagrams illustrating a transistor which is oneembodiment of the present invention.

FIGS. 11A and 11B are diagrams illustrating a display device which isone embodiment of the present invention.

FIG. 12 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIGS. 13A and 13B are diagrams each illustrating a display device whichis one embodiment of the present invention.

FIG. 14 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 15 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 16 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 17 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 18 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 19 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 20 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIG. 21 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIGS. 22A to 22C are diagrams each illustrating a display device whichis one embodiment of the present invention.

FIGS. 23A and 23B are diagrams illustrating a display device which isone embodiment of the present invention.

FIGS. 24A1, 24A2, and 24B are diagrams illustrating display deviceswhich are one embodiment of the present invention.

FIG. 25 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIGS. 26A and 26B are diagrams each illustrating an electronic devicewhich is one embodiment of the present invention.

FIGS. 27A and 27B are diagrams each illustrating an electronic devicewhich is one embodiment of the present invention.

FIGS. 28A and 28B are diagrams each illustrating an electronic devicewhich is one embodiment of the present invention.

FIGS. 29A to 29C are graphs for description of Example 1.

FIGS. 30A to 30C are graphs for description of Example 1.

FIGS. 31A to 31C are graphs for description of Example 1.

FIGS. 32A to 32C are graphs for description of Example 1.

FIG. 33 is a graph for description of Example 2.

FIG. 34 is a graph for description of Example 2.

FIG. 35 is a graph for description of Example 2.

FIG. 36 is a graph for description of Example 2.

FIGS. 37A to 37C are graphs for description of Example 2.

FIG. 38 is a graph for description of Example 2.

FIG. 39 is a graph for description of Example 2.

FIG. 40 is a graph for description of Example 2.

FIG. 41 is a graph for description of Example 2.

FIG. 42 is a graph for description of Example 3.

FIG. 43 is a graph for description of Example 3.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and examples of the present invention will be described withreference to the drawings. However, the present invention is not limitedto the description below, and those skilled in the art will appreciatethat a variety of modifications can be made to the modes and detailswithout departing from the spirit and scope of the present invention.Therefore, the present invention is not interpreted as being limited tothe following description in the embodiments and examples. Note that, inall the drawings for explaining the embodiments and examples, the sameportions or portions having the same functions are denoted by the samereference numerals, and the description thereof will be made only once.

Note that in Embodiments 1 to 4 which are described below, a transistorwhich is provided in at least a driver circuit portion of a displaydevice which is one embodiment of the present invention will bedescribed.

Embodiment 1

In this embodiment, a transistor that can be applied to a display devicewhich is one embodiment of the present invention and a manufacturingmethod thereof will be described. In a display device which is oneembodiment of the present invention, the transistor of this embodimentis provided in at least a driver circuit portion.

FIGS. 1A to 1C are cross-sectional views of transistors that can beapplied to one embodiment of the present invention.

A transistor 471 is a bottom-gate transistor, and includes a first gateelectrode layer 401, a gate insulating layer 402, an oxide semiconductorlayer 403, and source and drain electrode layers 405 which are providedover a substrate 400. In addition, a first protective insulating layer407 which is in contact with part of the oxide semiconductor layer 403and covers the first gate electrode layer 401, the gate insulating layer402, the oxide semiconductor layer 403, and the source and drainelectrode layers 405 is included, and a second gate electrode layer 409which is provided over the first protective insulating layer 407 andoverlaps with the oxide semiconductor layer 403 is included. Note thatthe first protective insulating layer 407 can be referred to as a secondgate insulating layer.

The oxide semiconductor layer 403 including a channel formation regionmay be formed using an oxide material having semiconductorcharacteristics. For example, an oxide semiconductor whose compositionformula is represented by InMO₃(ZnO)_(m) (m>0) can be used, andparticularly, an In—Ga—Zn—O-based oxide semiconductor is preferablyused. Note that M represents one or more metal elements selected fromGa, Fe, Ni, Mn, or Co. As an example, M may be Ga or may include theabove metal element in addition to Ga; for example, M may be Ga and Nior Ga and Fe.

Note that in the above oxide semiconductor, a transition metal elementsuch as Fe or Ni or an oxide of the transition metal may be contained inaddition to a metal element contained as M.

In this specification, an oxide semiconductor including a material whosecomposition formula is represented by InMO₃ (ZnO)_(m) (m>0) where atleast Ga is included as M is referred to as an In—Ga—Zn—O-based oxidesemiconductor, and a thin film thereof is also referred to as anIn—Ga—Zn—O-based non-single-crystal film.

As the oxide semiconductor applied to the oxide semiconductor layer 403,any of the following oxide semiconductors can be applied in addition tothe above: an In—Sn—Zn—O-based oxide semiconductor; an In—Al—Zn—O-basedoxide semiconductor; a Sn—Ga—Zn—O-based oxide semiconductor; anAl—Ga—Zn—O-based oxide semiconductor; a Sn—Al—Zn—O-based oxidesemiconductor; an In—Zn—O-based oxide semiconductor; a Sn—Zn—O-basedoxide semiconductor; an Al—Zn—O-based oxide semiconductor; an In—O-basedoxide semiconductor; a Sn—O-based oxide semiconductor; and a Zn—O-basedoxide semiconductor.

Further, silicon oxide may be included in the above oxide semiconductor.

The oxide semiconductor layer 403 can be formed in the following manner:at least after an oxide semiconductor film is formed, heat treatment(heat treatment for dehydration or dehydrogenation) through whichimpurities such as moisture (H₂O) are reduced is performed to reduce theresistance of the oxide semiconductor film (the carrier concentration ofthe oxide semiconductor film is increased, preferably to 1×10¹⁸/cm³ ormore); and the first protective insulating layer 407 is formed incontact with the oxide semiconductor film (or a processed oxidesemiconductor layer) so that the resistance of the oxide semiconductorfilm is raised (the carrier concentration of the oxide semiconductorfilm is decreased, preferably to less than 1×10¹⁸/cm³, more preferablyto 1×10¹⁴/cm³ or less). In such a manner, the oxide semiconductor layer403 that can be used as the channel formation region can be formed.

Further, after the heat treatment for dehydration or dehydrogenation isperformed so that impurities such as moisture are eliminated, the oxidesemiconductor layer is preferably slowly cooled (gradually cooled) underan inert atmosphere. After the oxide semiconductor layer is subjected toheat treatment for dehydration or dehydrogenation and is slowly cooled,an insulating oxide film or the like is formed in contact with the oxidesemiconductor layer; thus, the carrier concentration of the oxidesemiconductor layer can be reduced. In such a manner, the reliability ofthe transistor 471 can be improved.

Further, impurities such as moisture which are present not only in theoxide semiconductor layer 403, but also in the gate insulating layer402, at an interface between the oxide semiconductor layer 403 and alayer provided below and in contact with the oxide semiconductor layer403 (i.e., an interface between the oxide semiconductor layer 403 andthe gate insulating layer 402), and at an interface between the oxidesemiconductor layer 403 and a layer provided over and in contact withthe oxide semiconductor layer 403 (i.e., an interface between the oxidesemiconductor layer 403 and the first protective insulating layer 407)are reduced.

The oxide semiconductor layer 403 includes a high-resistance oxidesemiconductor region at least in a region which is in contact with aninorganic insulating film, and the high-resistance oxide semiconductorregion can serve as a channel formation region.

Note that an In—Ga—Zn—O-based non-single-crystal film used for the oxidesemiconductor layer 403 may be amorphous, microcrystalline, orpolycrystalline. Although the “In—Ga—Zn—O-based non-single-crystal film”is given, the oxide semiconductor layer 403 may be an In—Ga—Zn—O-basedsingle crystal film instead.

When the high-resistance oxide semiconductor region is used as a channelformation region, electric characteristics of the transistor can bestabilized and increase in off current or the like can be prevented.

The source and drain electrode layers 405 which are in contact with theoxide semiconductor layer 403 are preferably formed using a materialincluding a metal with high oxygen affinity. It is preferable that thematerial including a metal with high oxygen affinity be one or morematerials selected from titanium, aluminum, manganese, magnesium,zirconium, beryllium, or thorium.

When heat treatment is performed while the oxide semiconductor layer 403and the metal layer with high oxygen affinity are in contact with eachother, oxygen atoms move from the oxide semiconductor layer 403 to themetal layer, the carrier density in the vicinity of an interface isincreased, and a low-resistance region is formed. The low-resistanceregion may be in a film shape having an interface.

Through the above steps, the transistor whose contact resistance isreduced and on current is increased can be manufactured.

FIGS. 2A to 2D are cross-sectional views illustrating steps ofmanufacturing the transistor 471.

First, the first gate electrode layer 401 is formed over the substrate400 having an insulating surface. As the substrate 400 having aninsulating surface, any glass substrate used in the electronics industry(also called an alkali-free glass substrate) such as an aluminosilicateglass substrate, an aluminoborosilicate glass substrate, or a bariumborosilicate glass substrate, a plastic substrate with heat resistancewhich can withstand a process temperature in this manufacturing process,or the like can be used. When the substrate 400 having an insulatingsurface is a mother glass, any of the following sizes of the substratecan be used; the first generation (320 mm×400 mm), the second generation(400 mm×500 mm), the third generation (550 mm×650 mm), the fourthgeneration (680 mm×880 mm or 730 mm×920 mm), the fifth generation (1000mm×1200 mm or 1100 mm×1250 mm), the sixth generation (1500 mm×1800 mm),the seventh generation (1900 mm×2200 mm), the eighth generation (2160mm×2460 mm), the ninth generation (2400 mm×2800 mm or 2450 mm×3050 mm),the tenth generation (2950 mm×3400 mm), and the like.

Alternatively, as illustrated in FIG. 1C which will be described later,a base insulating layer may be formed between the substrate 400 and thefirst gate electrode layer 401. The base insulating layer may be formedto have a single-layer structure or a stacked-layer structure using aninsulating film that can prevent an impurity element (such as sodium)from diffusing from the substrate 400. For example, one or more filmsselected from a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, or a silicon oxynitride film can be used.

The first gate electrode layer 401 can be formed to have a single-layerstructure or a stacked-layer structure using a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material containing any of thesematerials as its main component.

For example, as a two-layer structure of the first gate electrode layer401, the following structures are preferable: a two-layer structure ofan aluminum layer and a molybdenum layer stacked thereover, a two-layerstructure of a copper layer and a molybdenum layer stacked thereover, atwo-layer structure of a copper layer and a titanium nitride layer or atantalum nitride layer stacked thereover, and a two-layer structure of atitanium nitride layer and a molybdenum layer. As a three-layerstructure, a stack of a tungsten layer or a tungsten nitride layer, alayer of an alloy of aluminum and silicon or an alloy of aluminum andtitanium, and a titanium nitride layer or a titanium layer ispreferable.

After a conductive film is formed over the entire surface of thesubstrate 400, a photolithography step is performed. A resist mask isformed over the conductive film, and an unnecessary portion is removedby etching. In such a manner, the first gate electrode layer 401 isformed. The first gate electrode layer 401 serves as a wiring and anelectrode (such as a gate wiring, a capacitor wiring, and a terminalelectrode which include the first gate electrode layer 401).

Next, the gate insulating layer 402 is formed over the first gateelectrode layer 401.

The gate insulating layer 402 can be formed to have a single-layerstructure or a stacked-layer structure using a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, or a silicon nitrideoxide layer by a plasma CVD method, a sputtering method, or the like.For example, a silicon oxynitride layer may be formed using SiH₄ and oneor both of oxygen and nitrogen as a source gas by a plasma CVD method.Alternatively, dinitrogen monoxide may be used instead of oxygen andnitrogen.

Next, an oxide semiconductor film is formed over the gate insulatinglayer 402.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust or the like on a surface of the gate insulating layer 402is preferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which an RF power source is used for application of voltage toa substrate under an argon atmosphere and plasma is generated to exposean object to be processed (e.g., the substrate) to the plasma so that asurface of the object is modified. Note that nitrogen, helium, or thelike may be used instead of an argon atmosphere. Alternatively, an argonatmosphere to which oxygen, dinitrogen monoxide, or the like is addedmay be used. Further alternatively, an argon atmosphere to whichchlorine, methane tetrafluoride, or the like is added may be used.

The oxide semiconductor film is formed using an In—Ga—Zn—O-based metaloxide as a target by a sputtering method. The oxide semiconductor filmcan be formed by a sputtering method under a rare gas (e.g., argon)atmosphere, an oxygen atmosphere, or an atmosphere including a rare gas(e.g., argon) and oxygen.

Note that the gate insulating layer 402 and the oxide semiconductor filmmay be formed successively without exposure to air. By successiveformation of the gate insulating layer 402 and the oxide semiconductorfilm without exposure to air, the interface between the gate insulatinglayer 402 and the oxide semiconductor film can be prevented from beingcontaminated by atmospheric components or impurities (such as moistureand hydrocarbon) floating in air, so that variation in characteristicsof the transistors can be reduced.

Next, the oxide semiconductor film is processed into an island-shapedfirst oxide semiconductor layer 430 by a photolithography step (see FIG.2A).

The first oxide semiconductor layer 430 is subjected to heat treatmentunder an inert gas (nitrogen or a rare gas such as helium, neon, orargon) atmosphere or reduced pressure, and is slowly cooled under aninert atmosphere, whereby a second oxide semiconductor layer 431 isformed (see FIG. 2B). When the first oxide semiconductor layer 430 issubjected to the heat treatment under the above atmosphere, impuritiessuch as hydrogen and moisture contained in the first oxide semiconductorlayer 430 can be removed, and the second oxide semiconductor layer 431can be formed.

It is preferable that impurities such as moisture and hydrogen be notcontained in nitrogen or a rare gas such as helium, neon, or argon inthe heat treatment. Alternatively, the purity of nitrogen or a rare gassuch as helium, neon, or argon introduced in a heat treatment apparatusis preferably 6 N (99.9999%) or higher, more preferably 7 N (99.99999%)or higher (that is, the concentration of the impurities is 1 ppm orlower, preferably 0.1 ppm or lower).

For the heat treatment, a method in which an electric furnace is used, agas rapid thermal anneal (GRTA) method in which a heated gas is used, aninstantaneous heating method such as a lamp rapid thermal anneal (LRTA)method in which lamp light is used, or the like can be used.

Here, the case where the first oxide semiconductor layer 430 issubjected to heat treatment in which an electric furnace is used will bedescribed with reference to FIG. 3.

FIG. 3 is a schematic view of an electric furnace 601. The electricfurnace 601 includes a chamber 602 and heaters 603 outside the chamber602. The heaters 603 are used for heating the chamber 602. Inside thechamber 602, a susceptor 605 in which a substrate 604 is mounted isprovided. The substrate 604 is transferred into/from the chamber 602.The chamber 602 is provided with a gas supply means 606 and anevacuation means 607. With the gas supply means 606, a gas is introducedinto the chamber 602. The evacuation means 607 exhausts the inside ofthe chamber 602 or reduces the pressure in the chamber 602. Note thatthe electric furnace 601 preferably has a structure in which thetemperature increases at greater than or equal to 0.1° C./min and lessthan or equal to 20° C./min and decreases at greater than or equal to0.1° C./min and less than or equal to 15° C./min.

The gas supply means 606 includes a gas supply source 611, a pressureadjusting valve 612, a mass flow controller 614, and a stop valve 615.In this embodiment, as illustrated in FIG. 3, it is preferable that arefining apparatus 613 be provided between the gas supply source 611 andthe chamber 602. The refining apparatus 613 can remove impurities suchas moisture and hydrogen in a gas which is introduced from the gassupply source 611 into the chamber 602; thus, entry into the chamber602, of moisture, hydrogen, and the like, can be prevented by provisionof the refining apparatus 613.

In this embodiment, nitrogen or a rare gas is introduced into thechamber 602 from the gas supply source 611, so that the inside of thechamber 602 is in a nitrogen or a rare gas atmosphere. In the chamber602 heated at greater than or equal to 200° C. and less than or equal to600° C., preferably, greater than or equal to 400° C. and less than orequal to 600° C., the first oxide semiconductor layer 430 formed overthe substrate 604 (the substrate 400 in FIGS. 1A to 1C) is heated,whereby the first oxide semiconductor layer 430 can be dehydrated ordehydrogenated.

Alternatively, the chamber 602 in which the pressure is reduced by theevacuation means 607 is heated at greater than or equal to 200° C. andless than or equal to 600° C., preferably, greater than or equal to 400°C. and less than or equal to 600° C. In such a chamber 602, the firstoxide semiconductor layer 430 formed over the substrate 604 (thesubstrate 400 in FIGS. 1A to 1C) is heated, whereby the first oxidesemiconductor layer 430 can be dehydrated or dehydrogenated.

Next, the heaters 603 are turned off, and the chamber 602 is slowlycooled (gradually cooled). By performance of heat treatment and slowcooling under an inert gas atmosphere or under reduced pressure,resistance of the first oxide semiconductor layer 430 is reduced (i.e.,the carrier concentration is increased, preferably to 1×10¹⁸/cm³ orhigher), so that a second oxide semiconductor layer 431 can be formed.

Through the heat treatment in the above-described manner, thereliability of the transistor formed later can be improved.

Note that in the case where heat treatment is performed under reducedpressure, an inert gas may be introduced into the chamber 602 after theheat treatment, so that the chamber 602 is to be under an atmosphericpressure, and then, cooling may be performed.

After the substrate 604 in the chamber 602 of the heating apparatus iscooled to about 300° C., the substrate 604 may be transferred into anatmosphere at room temperature. As a result, the cooling time of thesubstrate 604 can be shortened.

If the heating apparatus has a multi-chamber structure, heat treatmentand cool treatment can be performed in chambers different from eachother. For example, the first oxide semiconductor layer 430 over thesubstrate 604 (the substrate 400 in FIGS. 1A to 1C) is heated in a firstchamber which is filled with nitrogen or a rare gas and heated atgreater than or equal to 200° C. and less than or equal to 600° C.,preferably greater than or equal to 400° C. and less than or equal to600° C. Next, the substrate subjected to the heat treatment istransferred, through a transfer chamber in which nitrogen or a rare gasis introduced, into a second chamber which is filled with nitrogen or arare gas and heated at 100° C. or lower, preferably at room temperature,and then cooling treatment is performed therein. In such a manner, theheat treatment and the cooling treatment are performed in differentchambers, whereby throughput can be increased.

The heat treatment of the first oxide semiconductor layer 430 under aninert gas atmosphere or reduced pressure may be performed on the oxidesemiconductor film which has not yet been processed into theisland-shaped first oxide semiconductor layer 430. In that case, afterheat treatment of the oxide semiconductor film performed under an inertgas atmosphere or reduced pressure, slow cooling is performed to thetemperature equal to or higher than room temperature and lower than 100°C. Then, the substrate 604 (the substrate 400 in FIGS. 1A to 1C) istaken out from the heating apparatus, and a photolithography step isperformed.

The first oxide semiconductor layer 430 which has been subjected to heattreatment under an inert gas atmosphere or reduced pressure ispreferably in an amorphous state, but may be partly crystallized.

Next, a conductive film is formed over the gate insulating layer 402 andthe second oxide semiconductor layer 431.

As a material for the conductive film, an element selected fromaluminum, chromium, tantalum, titanium, molybdenum, or tungsten; analloy containing any of the above metal elements as its main component;an alloy containing the above metal elements in combination; and thelike can be given.

In the case where heat treatment is performed after formation of theconductive film, a conductive film having at least enough heatresistance to the heat treatment is used. For example, when theconductive film is formed using aluminum alone, there are disadvantagessuch as low resistance and a tendency to be corroded; therefore, theconductive film is formed using aluminum in combination with aconductive material having heat resistance. As the conductive materialhaving heat resistance which is used in combination with aluminum, anyof the following materials may be used: an element selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, orscandium, an alloy containing any of the above metal elements as itsmain component, an alloy containing the above elements in combination,and a nitride containing any of the above elements as its maincomponent.

The second oxide semiconductor layer 431 and the conductive film areetched, so that a third oxide semiconductor layer 432 and the source anddrain electrode layers 405 (a source electrode 405 a and a drainelectrode 405 b) are formed (see FIG. 2C). Note that part (a backchannel portion) of the third oxide semiconductor layer 432 is etched soas to have a groove (a depressed portion).

Next, the first protective insulating layer 407 is formed in contactwith the third oxide semiconductor layer 432. Moisture, hydrogen ions,OH⁻, and the like are reduced in the first protective insulating layer407, (that is, moisture, hydrogen ions, OH⁻, and the like are notincluded in the first protective insulating layer 407, or almost none ofthem are included in the first protective insulating layer 407). Thefirst protective insulating layer 407 can block entry of them from theoutside. The first protective insulating layer 407 is formed using aninsulating inorganic material containing oxygen, and specifically,silicon oxide, silicon oxynitride, or silicon nitride oxide ispreferably used.

In this embodiment, as the first protective insulating layer 407, a300-nm-thick silicon oxide film is formed by a sputtering method. Thesubstrate temperature in formation of the silicon oxide film may be fromroom temperature to 300° C. or lower and in this embodiment, is 100° C.The formation of the silicon oxide film by a sputtering method can beperformed under a rare gas (e.g., argon) atmosphere, an oxygenatmosphere, or an atmosphere of a mixed gas of a rare gas (e.g., argon)and oxygen. As a target, a silicon oxide target or a silicon target maybe used. For example, with use of a silicon target, a silicon oxide filmcan be formed by a sputtering method under an atmosphere containingoxygen.

When the oxide semiconductor film is formed as the first protectiveinsulating layer 407 by a sputtering method, a plasma CVD method, or thelike to be in contact with the third oxide semiconductor layer 432, inthe low-resistance third oxide semiconductor layer 432, at least aregion in contact with the first protective insulating layer 407 hasincreased resistance ((i.e., the carrier concentration is reduced,preferably to lower than 1×10¹⁸/cm³). Thus, a high-resistance oxidesemiconductor region can be formed.

During a manufacture process of the transistor, it is important toincrease and decrease the carrier concentration in the third oxidesemiconductor layer 432 through performance of heat treatment and slowcooling under an inert gas atmosphere (or reduced pressure), formationof an insulating oxide, and the like. The third oxide semiconductorlayer 432 becomes the oxide semiconductor layer 403 having ahigh-resistance oxide semiconductor region (see FIG. 2D).

Next, after a conductive film is formed over the first protectiveinsulating layer 407, a photolithography step is performed. A resistmask is formed over the conductive film, and an unnecessary portion isremoved by etching, so that the second gate electrode layer 409(including a wiring or the like which is formed using the same layer) isformed. When the second gate electrode layer 409 is selectively etchedso as to have a top surface having a desired shape, the first protectiveinsulating layer 407 can function as an etching stopper.

Note that in the case where the second gate electrode layer 409 isconnected to the first gate electrode layer 401, an opening whichexposes the first gate electrode layer 401 is formed in a predeterminedportion of the first protective layer 407 before the conductive filmwhich is to be the second gate electrode layer 409 is formed.

For the conductive film formed over the first protective insulatinglayer 407, a metal material (one or more of metal elements selected fromaluminum, copper, titanium, tantalum, tungsten, molybdenum, chromium,neodymium, or scandium, or an alloy containing any of the elements asits main component) can be used. A film formed using any of them canhave a light-blocking property when having a sufficient thickness. Insuch a manner, the oxide semiconductor layer 403 can be blocked fromlight.

In FIG. 1A, the width of the second gate electrode layer 409 is largerthan that of the first gate electrode layer 401 and larger than that ofthe oxide semiconductor layer 403. As illustrated in FIG. 1A, the widthof the second gate electrode layer 409 is made larger than that of theoxide semiconductor layer 403 so that the second gate electrode layer409 covers a top surface of the oxide semiconductor layer 403. In such amanner, the oxide semiconductor layer 403 can be blocked from light. Athin region of the oxide semiconductor layer 403 is not covered with thesource and drain electrode layers 405. Therefore, there is a possibilitythat the electric characteristics of the transistor 471 are influencedby light irradiation. For example, an In—Ga—Zn—O-basednon-single-crystal film formed by a sputtering method hasphotosensitivity at a wavelength of 450 nm or less; therefore, in thecase where an In—Ga—Zn—O-based non-single-crystal film is used for theoxide semiconductor layer 403, the second gate electrode layer 409 maybe provided so that light having a wavelength of 450 nm or less can beparticularly blocked.

Note that here, the transistor 471 may be subjected to heat treatmentunder a nitrogen atmosphere or an air atmosphere (in air). This heattreatment is preferably performed at a temperature of 300° C. or less,and the timing of the heat treatment is not particularly limited as longas it is performed after an insulating film which is to be the firstprotective insulating layer 407 is formed. For example, heat treatmentis performed at 350° C. for one hour under a nitrogen atmosphere. If theheat treatment is performed, variation in electric characteristics ofthe transistor 471 can be reduced.

Through the above steps, the transistor 471 illustrated in FIG. 1A canbe formed.

Note that the transistor used in this embodiment is not limited to theone of FIG. 1A. As illustrated in FIG. 1B, a planarizing layer (forexample, a resin layer) may be provided below a second gate electrodelayer 409B. FIG. 1B illustrates a structure in which a resin layer 408is formed between a second gate electrode layer 409B and the firstprotective insulating layer 407 which covers the first gate electrodelayer 401, the gate insulating layer 402, the oxide semiconductor layer403, and the source and drain electrode layers 405. By provision of theresin layer below the second gate electrode layer 409B, surfaceunevenness due to structures formed therebelow can be reduced, and asurface on which the second gate electrode layer 409B is formed can beplanarized. The method for planarization is not limited to the formationof the resin layer, and another method (such as a spin coating method ora reflow method) by which the top surface can be planarized may be used.

In FIG. 1B, the same portions as those of FIG. 1A other than differentportions are denoted by the same reference numerals.

The resin layer 408 covers the source and drain electrode layers 405 andthe oxide semiconductor layer 403 having the thin region with the firstprotective insulating layer 407 provided therebetween. The resin layer408 can be formed using, for example, a photosensitive ornon-photosensitive organic material to have a thickness of 0.5 μm to 3μm. As the photosensitive or non-photosensitive organic material usedfor the resin layer 408, polyimide, acrylic, polyamide, polyimideamide,resist, benzocyclobutene, or a stack of any of these materials is used.Here, a layer of photosensitive polyimide is formed by a coating methodas the resin layer 408. After polyimide is applied to the entiresurface, light exposure, development, and baking are performed, wherebythe resin layer 408 of polyimide whose surface is plane and has athickness of 1.5 μm is formed.

By provision of the resin layer 408, unevenness due to a structure of atransistor 471B can be reduced and the surface on which the second gateelectrode layer 409 is formed can be planarized.

FIG. 1C illustrates a structure in which a base insulating layer 410 isprovided between a first gate electrode layer 401C and the substrate 400over which the transistor is provided and the relationship between thewidth of the first gate electrode layer 401C and the width of a secondgate electrode layer 409C is different from that of FIG. 1A.

In FIG. 1C, the same portions as those of FIG. 1A other than differentportions are denoted by the same reference numerals.

The base insulating layer 410 is formed using a silicon oxynitridelayer, a silicon nitride oxide layer, a silicon nitride layer, or thelike having a thickness of 50 nm to 200 nm. In the case where glass isused as the substrate 400, the base insulating layer 410 can prevent animpurity element (such as sodium) in a glass substrate from diffusinginto a transistor 471C, in particular, can prevent such an impurityelement from entering the oxide semiconductor layer 403. In addition, inthe case where the base insulating layer 410 is provided, the substrate400 can be prevented from being etched in the etching step for formingthe first gate electrode layer 401C.

Note that in the transistor 471C, the relationship between the width ofa first gate electrode layer 401C and that of a second gate electrodelayer 409C are different from the relationship between the width of thefirst gate electrode layer and the second gate electrode layer of thetransistor 471 or the transistor 471B. The length of the first gateelectrode layer 401C in a channel length direction of the transistor471C in FIG. 1C is larger than that of the oxide semiconductor layer 403in the channel length direction. On the other hand, the length of thesecond gate electrode layer 409C in the channel length direction of thetransistor 471C is smaller than that of the oxide semiconductor layer403 in the channel length direction. As illustrated in FIG. 1C, thelength of the second gate electrode layer 409C in the channel lengthdirection is larger than at least the length of the thin region of theoxide semiconductor layer 403 (i.e., the region in contact with thefirst protective insulating layer 407), and the second gate electrodelayer 409C overlaps with the thin region of the oxide semiconductorlayer 403. In such a manner, when the length of the second gateelectrode layer 409C is small, parasitic capacitance can be reduced.

Note that in FIGS. 1A to 1C, before the first protective insulatinglayer 407 is formed, an exposed thin region of the oxide semiconductorlayer 403 may be subjected to oxygen radical treatment. By the oxygenradical treatment, an exposed surface and its vicinity of the oxidesemiconductor layer 403 can be modified into an oxygen-excess region,and can function as a high-resistance region. Oxygen radicals may besupplied by a plasma generating apparatus using a gas including oxygenor an ozone generating apparatus. The surface of the oxide semiconductorlayer 403 (the surface of a back channel portion) can be modified bybeing exposed to the supplied oxygen radicals or oxygen. The radicaltreatment is not limited to one using oxygen radicals, and may beperformed using argon and oxygen radicals. The treatment using argon andoxygen radicals is treatment in which an argon gas and an oxygen gas areintroduced to generate plasma, thereby modifying a surface of a thinfilm.

Note that in FIGS. 1A to 1C, the second gate electrode layer can beformed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

In FIGS. 1A to 1C, in the case where the second gate electrode layer isformed using a light-transmitting conductive material, the same materialas a pixel electrode is used for the second gate electrode layer, sothat the second gate electrode layer and the pixel electrode can beformed using the same photomask. When the second gate electrode layerand the pixel electrode are formed using the same material, the numberof steps can be reduced. In the case where the second gate electrodelayer is formed using a light-transmitting conductive material, alight-blocking layer for shielding the oxide semiconductor layer havingthe thin region from light is preferably separately formed at a positionoverlapping with the thin region of the oxide semiconductor layer. Amaterial and the thickness of the light-blocking layer are determinedsuch that the material has a light transmittance of at least less than50%, preferably less than 20% at a wavelength of 400 nm to 450 nm. Forexample, as a material of the light-blocking layer, a metal such aschromium (chromium oxide or chromium nitride may alternatively be used)or titanium nitride, or a black resin can be used. In the case of usinga black resin for blocking light, as the light intensity of light usedfor irradiation is higher, the light-blocking layer needs to be thicker.Therefore, in the case where the light-blocking layer needs to be thin,a metal film which has a high light-blocking property and can besubjected to a fine etching process and can be thinned is preferablyused.

Note that in the above description, an example in which a two-tonephotomask is used in a photolithography step is shown. When a resistmask including regions having different thicknesses (for example, twodifferent thicknesses of a two-tone mask) is used, the number of resistmasks can be reduced, so that the process can be simplified and cost canbe reduced. Note that in this specification, a gray-tone photomask and ahalf-tone photomask are collectively referred to as a multi-tone mask,for convenience. Note that the multi-tone mask is not limited to athree-tone mask, and a four-tone mask or a mask having five or moretones may be used.

In the case of using a multi-tone mask, after the oxide semiconductorfilm and the conductive film are stacked, a resist mask includingregions having different thicknesses is formed over the conductive film,and an oxide semiconductor layer having a thin region and source anddrain electrode layers are formed with use of the resist mask. In thiscase, end portions of the source and drain electrode layers and endportions of the oxide semiconductor layer are generally aligned witheach other, and side surfaces of the oxide semiconductor layer areexposed. Therefore, in the case where the first protective insulatinglayer 407 is formed, the side surfaces of the oxide semiconductor layerand the region (the thin region) of the oxide semiconductor layer whichdoes not overlap with the source and drain electrode layers are incontact with the first protective insulating layer 407.

The channel formation region in the semiconductor layer included in thetransistor of this embodiment is a high-resistance region; thus,electric characteristics of the transistor are stabilized and increasein off current can be prevented. Therefore, a display device including atransistor which has favorable electric characteristics and highreliability can be provided.

Note that this embodiment can be implemented in combination with any ofother embodiments described in this specification as appropriate.

Embodiment 2

In this embodiment, a transistor that can be applied to a display devicewhich is one embodiment of the present invention and that is differentfrom the transistor of Embodiment 1 and a manufacturing method thereofwill be described. In a display device which is one embodiment of thepresent invention, the transistor of this embodiment is provided in atleast a driver circuit portion.

FIGS. 4A and 4B each illustrate a cross-sectional view of a transistorwhich is one embodiment of the present invention. A transistor 472 is abottom-gate transistor, and includes a first gate electrode layer 401, agate insulating layer 402, an oxide semiconductor layer 403, n-typeoxide semiconductor layers 404, and source and drain electrode layers405 which are provided over a substrate 400. In addition, a firstprotective insulating layer 407 which is in contact with part of theoxide semiconductor layer 403 and covers the first gate electrode layer401, the gate insulating layer 402, the oxide semiconductor layer 403,the n-type oxide semiconductor layers 404, and the source and drainelectrode layers 405 is included, and a second gate electrode layer 409which is provided over the first protective insulating layer 407 andoverlaps with the oxide semiconductor layer 403 is included. Note thatthe first protective insulating layer 407 can be referred to as a secondgate insulating layer.

The n-type oxide semiconductor layers 404 having low resistance areprovided between the oxide semiconductor layer 403 and the source anddrain electrode layers 405, whereby the transistor 472 can operate morestably.

One example of a method for manufacturing the transistor 472 in FIG. 4Awill be described with reference to FIGS. 5A to 5D.

Note that steps in which the first gate electrode layer 401 is formedover the substrate 400 having an insulating surface, the gate insulatinglayer 402 covering the first gate electrode layer 401 is formed, and anoxide semiconductor film is formed are the same as those ofEmbodiment 1. Therefore, detailed description is omitted here and thesame portions as those of FIG. 1A are denoted by the same referencenumerals.

A first oxide semiconductor film 433 is formed over the gate insulatinglayer 402 as in Embodiment 1.

Next, a first n-type oxide semiconductor film 440 serving as source anddrain regions is formed over the first oxide semiconductor film 433 (seeFIG. 5A). The first n-type oxide semiconductor film 440 is formed usingan oxide semiconductor film having lower resistance than the first oxidesemiconductor film 433.

The first n-type oxide semiconductor film 440 may be formed using, forexample, an oxynitride film containing indium, gallium, and zinc whichis obtained by use of a metal oxide containing indium (In), gallium(Ga), and zinc (Zn) (In₂O₃:Ga₂O₃:ZnO=1:1:1) by a sputtering method underan atmosphere containing a nitrogen gas, an Al—Zn—O-basednon-single-crystal film, or an Al—Zn—O-based non-single-crystal filmcontaining nitrogen, i.e., an Al—Zn—O—N-based non-single-crystal film(also referred to as an AZON film).

Note that an In—Ga—Zn—O-based non-single-crystal film used in thisembodiment may be amorphous, microcrystalline, or polycrystalline.Alternatively, it may be a single crystal. By change in the condition offilm formation or composition ratio of a target in the above manner,crystalline states of the first oxide semiconductor film 433 and thefirst n-type oxide semiconductor film 440 can be changed.

Therefore, the crystalline states of the n-type oxide semiconductorlayers which are to be source and drain regions and the oxidesemiconductor layer 403 which forms a channel region may be differentfrom each other depending on the condition of the formation of the oxidesemiconductor film or the composition ratio of the target. For example,the n-type oxide semiconductor layers which are to be the source anddrain regions may include micro crystals; the oxide semiconductor layer403 may be amorphous; the n-type oxide semiconductor layers which are tobe the source and drain regions may be amorphous; or the oxidesemiconductor layer 403 may include micro crystals.

Note that the first oxide semiconductor film 433 and the first n-typeoxide semiconductor film 440 may be formed successively without beingexposed to air. Successive film formation without being exposed to airmakes it possible to obtain each interface between stacked layers, whichis not contaminated by atmospheric components or an impurity elementfloating in air, such as moisture, hydrocarbon, or the like. Therefore,variation in characteristics of the transistors can be reduced. Notethat the gate insulating layer 402, the first oxide semiconductor film433, and the first n-type oxide semiconductor film 440 may be formedsuccessively.

Next, as in Embodiment 1, the first oxide semiconductor film 433 issubjected to heat treatment. By performance of heat treatment and slowcooling under an inert gas atmosphere or under reduced pressure,resistance of the first oxide semiconductor film 433 is reduced (i.e.,the carrier concentration is increased, preferably to 1×10¹⁸/cm³ orhigher), so that a low-resistance oxide semiconductor film (a secondn-type oxide semiconductor film) can be formed.

The first oxide semiconductor film 433 is subjected to heat treatmentunder an inert gas (nitrogen or a rare gas such as helium, neon, orargon) atmosphere or reduced pressure. By the heat treatment under theabove atmosphere on the first oxide semiconductor film 433, impuritiessuch as hydrogen and moisture contained in the first oxide semiconductorfilm 433 can be removed.

It is preferable that impurities such as moisture and hydrogen be notcontained in nitrogen or a rare gas such as helium, neon, or argon inthe heat treatment. Alternatively, the purity of nitrogen or a rare gassuch as helium, neon, or argon introduced in a heat treatment apparatusis preferably 6 N (99.9999%) or higher, more preferably 7 N (99.99999%)or higher (that is, the concentration of the impurities is 1 ppm orlower, preferably 0.1 ppm or lower).

In this embodiment, an electric furnace has a structure in which thetemperature increases at greater than or equal to 0.1° C./min and lessthan or equal to 20° C./min, the atmosphere in the chamber is a nitrogenatmosphere or a rare gas atmosphere, and the temperature is set atgreater than or equal to 200° C. and less than or equal to 600° C.,preferably greater than or equal to 400° C. and less than or equal to600° C. In such a manner, the first oxide semiconductor film 433 and thefirst n-type oxide semiconductor film 440 which are formed over thesubstrate are heated. Alternatively, the pressure is reduced by anevacuation means, and the temperature is set at greater than or equal to200° C. and less than or equal to 600° C., preferably greater than orequal to 400° C. and less than or equal to 600° C. In such a manner, thefirst oxide semiconductor film 433 and the first n-type oxidesemiconductor film 440 which are formed over the substrate are heated,so that a second oxide semiconductor film and a second n-type oxidesemiconductor film are formed.

After the heat treatment, the heaters of the electric furnace are turnedoff, so that the chamber is slowly cooled (gradually cooled). Note thatthe electric furnace preferably has a structure in which the temperaturedecreases at greater than or equal to 0.1° C./min and less than or equalto 15° C./min.

Through the heat treatment in the above-described manner, thereliability of the transistor formed later can be improved.

Next, a resist mask (not shown) is formed over the second oxidesemiconductor film and the second n-type oxide semiconductor film by aphotolithography step, and the films are processed into an island-shapedsecond oxide semiconductor layer 431 and a second n-type oxidesemiconductor layer 434 by an etching step (see FIG. 5B).

Note that here, the oxide semiconductor films are processed after theheat treatment; however, the heat treatment may be performed after theoxide semiconductor films are processed.

Next, after the resist mask is removed, a conductive film is formed overthe second n-type oxide semiconductor layer 434.

As a material for the conductive film, an element selected fromaluminum, chromium, tantalum, titanium, molybdenum, or tungsten; analloy containing any of the above metal elements as its component; analloy containing the above metal elements in combination; and the likecan be given.

If heat treatment is performed after formation of the conductive film, aconductive film having at least enough heat resistance to the heattreatment is used.

Next, a photolithography step is performed. A resist mask is formed overthe conductive film, and the conductive film is etched, whereby thesource and drain electrode layers 405 are formed. Note that the secondn-type oxide semiconductor layer 434 in a region between source anddrain electrodes which are formed by the source and drain electrodelayers 405 (i.e., a back channel portion) is etched with use of the sameresist mask, so that second n-type oxide semiconductor layers 437 whichare to be source and drain regions are formed (see FIG. 5C). Note thatonly part of the second oxide semiconductor layer 431 is etched to be athird oxide semiconductor layer 432 having a groove (a recessedportion).

Next, the first protective insulating layer 407 is formed using aninorganic insulating film containing oxygen, such as silicon oxide orsilicon nitride oxide, in contact with the third oxide semiconductorlayer 432. Here, as in Embodiment 1, a silicon oxide film having athickness of 300 nm is formed by a sputtering method as the firstprotective insulating layer 407.

When the first protective insulating layer 407 is formed by a sputteringmethod, a plasma CVD method, or the like to be in contact with thelow-resistance first oxide semiconductor layer 432 with use of siliconoxide, in the low-resistance third oxide semiconductor layer 432, atleast a region in contact with the first protective insulating layer 407has increased resistance (i.e., the carrier concentration is reduced,preferably to lower than 1×10¹⁸/cm³). Thus, a high-resistance oxidesemiconductor region can be formed.

During a manufacture process of the transistor, it is important toincrease and decrease the carrier concentration in the third oxidesemiconductor layer 432 through performance of heat treatment and slowcooling under an inert gas atmosphere (or reduced pressure), formationof an insulating oxide, and the like. The third oxide semiconductorlayer 432 becomes the oxide semiconductor layer 403 having ahigh-resistance oxide semiconductor region (see FIG. 5D).

Note that steps after formation of the first protective insulating layer407 are the same as those of Embodiment 1. That is, the second gateelectrode layer 409 is formed over the first protective insulating layer407.

Note that a resin layer may be provided over the second gate electrodelayer 409. By provision of the resin layer over the second gateelectrode layer 409, unevenness due to a structure of the transistor 472can be reduced and the element can be planarized.

Note that the transistor 472 may be subjected to heat treatment under anitrogen atmosphere or an air atmosphere (in air). This heat treatmentis preferably performed at a temperature of 300° C. or less, and thetiming of the heat treatment is not particularly limited as long as itis performed after an insulating film which is to be the firstprotective insulating layer 407 is formed. For example, heat treatmentis performed at 350° C. for one hour under a nitrogen atmosphere. If theheat treatment is performed, variation in electric characteristics ofthe transistor 472 can be reduced.

Through the above steps, the transistor 472 illustrated in FIG. 4A canbe formed. Note that in the transistor 472, the first protectiveinsulating layer 407 functions as a second gate insulating layer.

FIG. 4B illustrates a structure in which a resin layer 408 is formedbetween the second gate electrode layer 409 and the first protectiveinsulating layer 407 which covers the first gate electrode layer 401,the gate insulating layer 402, the oxide semiconductor layer 403, then-type oxide semiconductor layers 404, and the source and drainelectrode layers 405.

A transistor 472B in FIG. 4B has a structure which is partly differentfrom that of FIG. 4A. In FIG. 4B, the same portions as those of FIG. 4Aother than different portions are denoted by the same referencenumerals.

The resin layer 408 covers the source and drain electrode layers 405 andthe oxide semiconductor layer 403 having the thin region with the firstprotective insulating layer 407 provided therebetween. The resin layer408 can be formed using a photosensitive or non-photosensitive organicmaterial to have a thickness of 0.5 μm to 3 μm. As the photosensitive ornon-photosensitive organic material used for the resin layer 408,polyimide, acrylic, polyamide, polyimideamide, resist, benzocyclobutene,or a stack of any of these materials is used. Here, a layer ofphotosensitive polyimide is formed by a coating method as the resinlayer 408. After polyimide is applied to the entire surface, lightexposure, development, and baking are performed, whereby the resin layer408 of polyimide whose surface is plane and has a thickness of 1.5 μm isformed.

By provision of the resin layer 408, unevenness due to a structure ofthe transistor 472B can be reduced and the surface on which the secondgate electrode layer 409 is formed can be planarized.

Note that as illustrated in FIG. 4A, the width of the second gateelectrode layer 409 is made larger than that of the first gate electrodelayer 401 and that of the oxide semiconductor layer 403, whereby theoxide semiconductor layer 403 can be shielded from light by the secondgate electrode layer 409. Gate voltage can be applied to the entireoxide semiconductor layer 403 from the second gate electrode layer 409.

Note that even if the structure of FIG. 4A or FIG. 4B is employed, inthe case where a portion in which the first protective insulating layer407 and the resin layer 408 are stacked is thin, a problem of parasiticcapacitance between the second gate electrode layer 409 and the sourceand drain electrode layers 405 arises in some cases. In the case where aproblem of parasitic capacitance arises, the width of the second gateelectrode layer 409 is preferably made small so that the area where thesecond gate electrode layer 409 and the source and drain electrodelayers 405 overlap with each other can be reduced. When the area wherethey overlap with each other is reduced, parasitic capacitance can bereduced.

Note that in the case where parasitic capacitance does not become aproblem because the portion in which the resin layer 408 and the firstprotective insulating layer 407 are stacked is sufficiently thick, thesecond gate electrode may be used as a common gate electrode whichcovers a plurality of transistors in the driver circuit and may have anarea substantially the same or larger than the area of the drivercircuit.

Note that in the above description, an example in which a two-tonephotomask is used in a photolithography step is shown. When a resistmask including regions having different thicknesses (for example, twodifferent thicknesses of a two-tone mask) is used, the number of resistmasks can be reduced, so that the process can be simplified and cost canbe reduced.

In the case of using a multi-tone mask, after the oxide semiconductorfilm of two stacked layers and the conductive film are stacked, a resistmask including regions having different thicknesses is formed over theconductive film, and an oxide semiconductor layer having a thin regionand source and drain electrode layers are formed with use of the resistmask. In this case, end portions of the source and drain electrodelayers and end portions of the oxide semiconductor layer are generallyaligned with each other, and side surfaces of the oxide semiconductorlayer are exposed. Therefore, in the case where the first protectiveinsulating layer 407 is formed, the side surfaces and the region (thethin region) which does not overlap with the source and drain electrodelayers of the oxide semiconductor layer are in contact with the firstprotective insulating layer 407.

The channel formation region in the semiconductor layer included in thetransistor of this embodiment is a high-resistance region; thus,electric characteristics of the transistor are stabilized and increasein off current can be prevented. Therefore, a semiconductor device (adisplay device) including a transistor which has favorable electriccharacteristics and high reliability can be provided.

Note that this embodiment can be implemented in combination with any ofother embodiments described in this specification as appropriate.

Embodiment 3

In this embodiment, a transistor that can be applied to a display devicewhich is one embodiment of the present invention and that is differentfrom the transistors of Embodiments 1 and 2 and a manufacturing methodthereof will be described. In a display device which is one embodimentof the present invention, the transistor of this embodiment is providedin at least a driver circuit portion.

FIGS. 6A and 6B each illustrate a cross-sectional view of a transistorwhich is one embodiment of the present invention. A transistor 473 is abottom-gate transistor, and includes a first gate electrode layer 401, agate insulating layer 402, an oxide semiconductor layer 403, source anddrain electrode layers 405 (a source electrode 405 a and a drainelectrode 405 b), and a channel protective layer 406 which are providedover a substrate 400. In addition, a first protective insulating layer407 which is in contact with the channel protective layer 406 and coversthe first gate electrode layer 401, the gate insulating layer 402, theoxide semiconductor layer 403, and the source and drain electrode layers405 is included, and a second gate electrode layer 409 which is formedover the first protective insulating layer 407 and overlaps with theoxide semiconductor layer 403 is included. That is, the transistor 473described in this embodiment is a channel-stop-type transistor.

One example of a method for manufacturing the transistor 473 in FIG. 6Awill be described with reference to FIGS. 7A to 7D.

Note that steps in which the first gate electrode layer 401 is formedover the substrate 400 having an insulating surface, the gate insulatinglayer 402 covering the first gate electrode layer 401 is formed, and anoxide semiconductor film is formed are the same as those ofEmbodiment 1. Therefore, detailed description is omitted here and thesame portions as those of FIG. 2A are denoted by the same referencenumerals.

A first oxide semiconductor film is formed over the gate insulatinglayer 402 as in Embodiment 1.

Next, a photolithography step is performed. A resist mask is formed overthe first oxide semiconductor film, and the first oxide semiconductorfilm is etched, so that an island-shaped oxide semiconductor layer 430is formed. Note that etching here is not limited to wet etching and maybe dry etching (see FIG. 7A).

Next, as in Embodiment 1, the first oxide semiconductor layer 430 issubjected to heat treatment. By performance of heat treatment and slowcooling under an inert gas atmosphere or under reduced pressure,resistance of the first oxide semiconductor layer 430 is reduced (i.e.,the carrier concentration is increased, preferably to 1×10¹⁸/cm³ orhigher), so that a low-resistance second oxide semiconductor layer 431can be formed.

The first oxide semiconductor layer 430 is subjected to heat treatmentunder an inert gas (nitrogen or a rare gas such as helium, neon, orargon) atmosphere or reduced pressure. By the heat treatment under theabove atmosphere on the first oxide semiconductor layer 430, impuritiessuch as hydrogen and moisture contained in the first oxide semiconductorlayer 430 can be removed.

It is preferable that impurities such as moisture and hydrogen be notcontained in nitrogen or a rare gas such as helium, neon, or argon inthe heat treatment. Alternatively, the purity of nitrogen or a rare gassuch as helium, neon, or argon introduced in a heat treatment apparatusis preferably 6 N (99.9999%) or higher, more preferably 7 N (99.99999%)or higher (that is, the concentration of the impurities is 1 ppm orlower, preferably 0.1 ppm or lower).

In this embodiment, an electric furnace has a structure in which thetemperature increases at greater than or equal to 0.1° C./min and lessthan or equal to 20° C./min, the atmosphere in the chamber is a nitrogenatmosphere or a rare gas atmosphere, and the temperature is set atgreater than or equal to 200° C. and less than or equal to 600° C.,preferably greater than or equal to 400° C. and less than or equal to600° C. In such a manner, the first oxide semiconductor layer 430 whichis formed over the substrate is heated in the heated chamber.Alternatively, the pressure is reduced by an evacuation means, and thetemperature is set at greater than or equal to 200° C. and less than orequal to 600° C., preferably greater than or equal to 400° C. and lessthan or equal to 600° C. In such a manner, the first oxide semiconductorlayer 430 which is formed over the substrate is heated, so that thesecond oxide semiconductor layer 431 is formed.

After the heat treatment, the heaters of the electric furnace are turnedoff, so that the chamber is slowly cooled (gradually cooled). Note thatthe electric furnace preferably has a structure in which the temperaturedecreases at greater than or equal to 0.1° C./min and less than or equalto 15° C./min.

Through the heat treatment in the above-described manner, thereliability of the transistor formed later can be improved.

Next, an insulating film which is to be a channel protective layer isformed in contact with the second oxide semiconductor layer 431.Moisture, hydrogen ions, OH⁻, and the like are reduced in the insulatingfilm which is to be the channel protective layer and is formed incontact with the second oxide semiconductor layer, and are preventedfrom entering the insulating film from the outside. The insulating filmis formed using an insulating inorganic material containing oxygen.Specifically, silicon oxide, silicon oxynitride, or silicon nitrideoxide is used. That is, the insulating film which is to be the channelprotective layer may be formed in a manner similar to formation of thefirst protective insulating layer 407 described in Embodiment 1.

In this embodiment, as the insulating film which is to be the channelprotective layer, a 300-nm-thick silicon oxide film is formed by asputtering method. The substrate temperature in film formation may befrom room temperature to 300° C. or lower and in this embodiment, is100° C. The formation of the silicon oxide film by a sputtering methodcan be performed under a rare gas (e.g., argon) atmosphere, an oxygenatmosphere, or an atmosphere of a mixed gas of a rare gas (e.g., argon)and oxygen. As a target, a silicon oxide target or a silicon target maybe used. For example, with use of a silicon target, a silicon oxide filmcan be formed by a sputtering method under an atmosphere containingoxygen.

When the insulating film which is to be the channel protective layer isformed by a sputtering method, a plasma CVD method, or the like to be incontact with the second oxide semiconductor layer 431, in thelow-resistance second oxide semiconductor layer 431, at least a regionin contact with the insulating film which is to be the channelprotective layer has increased resistance ((i.e., the carrierconcentration is reduced, preferably to lower than 1×10¹⁸/cm³). Thus, ahigh-resistance oxide semiconductor region can be formed.

During a manufacture process of the transistor, it is important toincrease and decrease the carrier concentration in the third oxidesemiconductor layer through performance of heat treatment and slowcooling under an inert gas atmosphere (or reduced pressure), formationof an insulating oxide, and the like. The second oxide semiconductorlayer 431 becomes the oxide semiconductor layer 403 having ahigh-resistance oxide semiconductor region.

Next, a photolithography step is performed. A resist mask is formed overthe insulating film that is to be the channel protective layer, and anunnecessary portion is removed by etching, so that the channelprotective layer 406 is formed. Note that the width of the first gateelectrode layer 401 is preferably larger than that of the channelprotective layer 406 (i.e., the length of the channel protective layer406 in the channel length direction) (see FIG. 7B).

Next, after the resist mask is removed, a conductive film is formed overthe second oxide semiconductor layer 431 and the channel protectivelayer 406.

As a material for the conductive film, an element selected fromaluminum, chromium, tantalum, titanium, molybdenum, or tungsten; analloy containing any of the above metal elements as its component; analloy containing the above metal elements in combination; and the likecan be given.

If heat treatment is performed after formation of the conductive film, aconductive film having at least enough heat resistance to the heattreatment is used.

Next, a photolithography step is performed. A resist mask is formed overthe conductive film, and the conductive film is etched, whereby thesource and drain electrode layers 405 (the source electrode 405 a and adrain electrode 405 b) are formed. In this etching, the channelprotective layer 406 functions as an etching stopper of the oxidesemiconductor layer 403. Therefore, the oxide semiconductor layer 403 isnot etched.

Because of the structure in which the channel protective layer 406 isprovided on and in contact with a channel formation region of the oxidesemiconductor layer 403, damage to the channel formation region of theoxide semiconductor layer 403 (for example, reduction in film thicknessdue to plasma or an etchant in etching, or oxidation) in themanufacturing process can be prevented. Therefore, the reliability ofthe transistor 473 can be improved.

Next, the first protective insulating layer 407 is formed over thesource and drain electrode layers 405 and the channel protective layer406. Moisture, hydrogen ions, OH⁻, and the like are reduced in the firstprotective insulating layer 407, and are prevented from entering thefirst protective insulating layer 407 from the outside. The firstprotective insulating layer 407 is formed using an insulating inorganicmaterial containing oxygen. Specifically, silicon oxide, siliconnitride, silicon oxynitride, silicon nitride oxide, aluminum oxide,aluminum nitride, magnesium oxide, yttrium oxide, hafnium oxide, ortantalum oxide can be given (see FIG. 7D).

Note that steps after formation of the first protective insulating layer407 are the same as those of Embodiment 1. That is, the second gateelectrode layer 409 is formed over the first protective insulating layer407.

Note that a resin layer may be provided over the second gate electrodelayer 409. By provision of the resin layer over the second gateelectrode layer 409, unevenness due to a structure of the transistor 473can be reduced and the element can be planarized.

Note that the transistor 473 may be subjected to heat treatment under anitrogen atmosphere or an air atmosphere (in air). This heat treatmentis performed at a temperature of 300° C. or less, and the timing of theheat treatment is not particularly limited as long as it is performedafter the channel protective layer 406 is formed. For example, heattreatment is performed at 350° C. for one hour under a nitrogenatmosphere. If the heat treatment is performed, variation in electriccharacteristics of the transistor 473 can be reduced.

Through the above steps, the transistor 473 illustrated in FIG. 6A canbe formed. Note that in the transistor 473, a portion in which thechannel protective layer 406 and the first protective insulating layer407 are stacked functions as a second gate insulating layer.

A transistor 473B in FIG. 6B has a structure which is partly differentfrom that of FIG. 6A. In FIG. 6B, the same portions as those of FIG. 6Aother than different portions are denoted by the same referencenumerals.

FIG. 6B illustrates a structure in which a resin layer 408 is formedbetween the second gate electrode layer 409 the first protectiveinsulating layer 407 which covers the first gate electrode layer 401,the gate insulating layer 402, the oxide semiconductor layer 403, andthe source and drain electrode layers 405.

The resin layer 408 covers the source and drain electrode layers 405 andthe channel protective layer 406 with the first protective insulatinglayer 407 provided therebetween. The resin layer 408 can be formed usinga photosensitive or non-photosensitive organic material to have athickness of 0.5 μm to 3 μm. As the photosensitive or non-photosensitiveorganic material used for the resin layer 408, polyimide, acrylic,polyamide, polyimideamide, resist, benzocyclobutene, or a stack of anyof these materials is used. Here, a layer of photosensitive polyimide isformed by a coating method as the resin layer 408. After polyimide isapplied to the entire surface, light exposure, development, and bakingare performed, whereby the resin layer 408 of polyimide whose surface isplane and has a thickness of 1.5 μm can be formed.

By provision of the resin layer 408, unevenness due to a structure ofthe transistor 473B can be reduced and the surface on which the secondgate electrode layer 409 is formed can be planarized.

Note that as illustrated in FIG. 6A, the width of the second gateelectrode layer 409 is made larger than that of the gate electrode layer401 and that of the oxide semiconductor layer 403, whereby gate voltagecan be applied to the entire oxide semiconductor layer 403 from thesecond gate electrode layer 409.

Note that even if the structure of FIG. 6A or FIG. 6B is employed, inthe case where a portion in which the channel protective layer 406, thefirst protective insulating layer 407, and the resin layer 408 arestacked is thin, a problem of parasitic capacitance between the secondgate electrode layer 409 and the source and drain electrode layers 405arises in some cases. In the case where a problem of parasiticcapacitance arises, the width of the second gate electrode layer 409 ismade smaller than that of the first gate electrode layer 401, and thearea where the second gate electrode layer 409 and the source and drainelectrode layers 405 overlap with each other is preferably reduced. Whenthe area where they overlap with each other is reduced, parasiticcapacitance can be reduced. Further, the width of the first gateelectrode layer 401 may be set to be smaller than that of the channelprotective layer 406 and the width of the second gate electrode layer409 may be set to be smaller than that of the channel protective layer406 so that the second gate electrode 409 does not overlap with thesource and drain electrode layers 405, whereby more parasiticcapacitance may be reduced.

Note that in the case where parasitic capacitance does not become aproblem because the portion in which the resin layer 408 and the firstprotective insulating layer 407 are stacked is sufficiently thick, thesecond gate electrode may be used as a common gate electrode whichcovers a plurality of transistors in the driver circuit and may have anarea substantially the same or larger than the area of the drivercircuit.

The channel formation region in the semiconductor layer included in thetransistor of this embodiment is a high-resistance region; thus,electric characteristics of the transistor are stabilized and increasein off current can be prevented. Therefore, a semiconductor device (adisplay device) including a transistor which has favorable electriccharacteristics and high reliability can be provided.

Note that this embodiment can be implemented in combination with any ofother embodiments described in this specification as appropriate.

Embodiment 4

In this embodiment, a transistor that can be applied to a display devicewhich is one embodiment of the present invention and that is differentfrom the transistors of Embodiments 1 to 3 and a manufacturing methodthereof will be described. In a display device which is one embodimentof the present invention, the transistor of this embodiment is providedin at least a driver circuit portion.

FIGS. 8A and 8B each illustrate a cross-sectional view of a transistorwhich is one embodiment of the present invention. A transistor 474 is abottom-gate transistor, and includes a first gate electrode layer 401, agate insulating layer 402, an oxide semiconductor layer 403, n-typeoxide semiconductor layers 404 a and 404 b, source and drain electrodelayers 405 (a source electrode 405 a and a drain electrode 405 b), and achannel protective layer 406 which are provided over a substrate 400. Inaddition, a first protective insulating layer 407 which is in contactwith the channel protective layer 406 and covers the first gateelectrode layer 401, the gate insulating layer 402, the oxidesemiconductor layer 403, the n-type oxide semiconductor layers 404 a and404 b, and the source and drain electrode layers 405 (the sourceelectrode 405 a and the drain electrode 405 b) is included, and a secondgate electrode layer 409 which is formed over the first protectiveinsulating layer 407 and overlaps with the oxide semiconductor layer 403is included. That is, the transistor 474 described in this embodiment isa channel-stop-type transistor.

One example of a method for manufacturing the transistor 474 in FIG. 8Awill be described with reference to FIGS. 9A to 9D.

Note that steps in which the first gate electrode layer 401 is formedover the substrate 400 having an insulating surface, the gate insulatinglayer 402 covering the first gate electrode layer 401 is formed, and anoxide semiconductor film is formed are the same as those of Embodiment3. Therefore, detailed description is omitted here and the same portionsas those of FIG. 7A are denoted by the same reference numerals.

A first oxide semiconductor film 433 is formed over the gate insulatinglayer 402 as in Embodiment 1.

Next, as in Embodiment 1, the first oxide semiconductor film 433 issubjected to heat treatment. By performance of heat treatment and slowcooling under an inert gas atmosphere or under reduced pressure,resistance of the first oxide semiconductor film 433 is reduced (i.e.,the carrier concentration is increased, preferably to 1×10¹⁸/cm³ orhigher), so that a low-resistance second oxide semiconductor film can beformed.

The first oxide semiconductor film 433 is subjected to heat treatmentunder an inert gas (nitrogen or a rare gas such as helium, neon, orargon) atmosphere or reduced pressure. By the heat treatment under theabove atmosphere on the first oxide semiconductor film 433, impuritiessuch as hydrogen and moisture contained in the first oxide semiconductorfilm 433 can be removed.

It is preferable that impurities such as moisture and hydrogen be notcontained in nitrogen or a rare gas such as helium, neon, or argon inthe heat treatment. Alternatively, the purity of nitrogen or a rare gassuch as helium, neon, or argon introduced in a heat treatment apparatusis preferably 6 N (99.9999%) or higher, more preferably 7 N (99.99999%)or higher (that is, the concentration of the impurities is 1 ppm orlower, preferably 0.1 ppm or lower).

In this embodiment, an electric furnace has a structure in which thetemperature increases at greater than or equal to 0.1° C./min and lessthan or equal to 20° C./min, the atmosphere in the chamber is a nitrogenatmosphere or a rare gas atmosphere, and the temperature is set atgreater than or equal to 200° C. and less than or equal to 600° C.,preferably greater than or equal to 400° C. and less than or equal to600° C. In such a manner, the first oxide semiconductor film 433 whichis formed over the substrate is heated in the heated chamber.

After the heat treatment, the heaters of the electric furnace are turnedoff, so that the chamber is slowly cooled (gradually cooled). Note thatthe electric furnace preferably has a structure in which the temperaturedecreases at greater than or equal to 0.1° C./min and less than or equalto 15° C./min.

Through the heat treatment in the above-described manner, thereliability of the transistor formed later can be improved.

Next, an insulating film which is to be a channel protective layer isformed in contact with the second oxide semiconductor film. Moisture,hydrogen ions, OH⁻, and the like are reduced in the insulating filmwhich is to be the channel protective layer and is formed in contactwith the second oxide semiconductor film, and are prevented fromentering the insulating film from the outside. The insulating film isformed using an insulating inorganic material containing oxygen.Specifically, a silicon oxide film or a silicon nitride oxide film isused.

In this embodiment, as the insulating film which is to be the channelprotective layer, a 300-nm-thick silicon oxide film is formed by asputtering method. The substrate temperature in film formation may befrom room temperature to 300° C. or lower and in this embodiment, is100° C. The formation of the silicon oxide film by a sputtering methodcan be performed under a rare gas (e.g., argon) atmosphere, an oxygenatmosphere, or an atmosphere of a mixed gas of a rare gas (e.g., argon)and oxygen. As a target, a silicon oxide target or a silicon target maybe used. For example, with use of a silicon target, a silicon oxide filmcan be formed by a sputtering method under an atmosphere containingoxygen.

When the insulating film which is to be the channel protective layer isformed by a sputtering method, a plasma CVD method, or the like to be incontact with the second oxide semiconductor film, in the low-resistancesecond oxide semiconductor film, at least a region in contact with theinsulating film which is to be the channel protective layer hasincreased resistance ((i.e., the carrier concentration is reduced,preferably to lower than 1×10¹⁸/cm³). Thus, a high-resistance oxidesemiconductor region can be formed.

During a manufacture process of the transistor, it is important toincrease and decrease the carrier concentration in the oxidesemiconductor layer through performance of heat treatment and slowcooling under an inert gas atmosphere (or reduced pressure), formationof an insulating oxide, and the like. The second oxide semiconductorfilm becomes the third oxide semiconductor film having a high-resistanceoxide semiconductor region.

Next, a photolithography step is performed. A resist mask is formed overthe insulating film that is to be the channel protective layer, and anunnecessary portion is removed by etching, so that the channelprotective layer 406 is formed. Note that the width of the first gateelectrode layer 401 is preferably larger than that of the channelprotective layer 406 (i.e., the length of the channel protective layer406 in the channel length direction).

Next, an n-type oxide semiconductor film serving as source and drainregions is formed over the third oxide semiconductor film and thechannel protective layer 406. The n-type oxide semiconductor film isformed using an oxide semiconductor film having lower resistance thanthe third oxide semiconductor film.

The n-type oxide semiconductor film may be formed using, for example, anoxynitride film containing indium, gallium, and zinc which is obtainedby use of a metal oxide containing indium (In), gallium (Ga), and zinc(Zn) (In₂O₃:Ga₂O₃:ZnO=1:1:1) by a sputtering method under an atmospherecontaining a nitrogen gas, an Al—Zn—O-based non-single-crystal film, oran Al—Zn—O-based non-single-crystal film containing nitrogen, i.e., anAl—Zn—O—N-based non-single-crystal film (also referred to as an AZONfilm).

Note that an In—Ga—Zn—O-based non-single-crystal film used in thisembodiment may be amorphous, microcrystalline, or polycrystalline.Alternatively, the crystalline state is not limited thereto, and it maybe a single crystal. By change in the condition of film formation orcomposition ratio of a target in the above manner, crystalline states ofthe third oxide semiconductor film and the n-type oxide semiconductorfilm can be changed.

Therefore, the crystalline states of the n-type oxide semiconductor filmwhich is to be the source and drain regions and the third oxidesemiconductor film which forms a channel region may be different fromeach other depending on the condition of the formation of the oxidesemiconductor film or the composition ratio of the target. For example,the n-type oxide semiconductor film which is to be the source and drainregions may include micro crystals; the third oxide semiconductor filmmay be amorphous; the n-type oxide semiconductor film which is to be thesource and drain regions may be amorphous; or the third oxidesemiconductor film may include micro crystals.

Next, a photolithography step is performed. A resist mask is formed overthe n-type oxide semiconductor film, and an unnecessary portion of then-type oxide semiconductor film and the third oxide semiconductor filmis removed by etching. In such a manner, the oxide semiconductor layer403 is formed (see FIG. 9B).

Note that the photolithography step is not particularly limited to theabove description, and the following manner may be alternativelyemployed: a resist mask is formed over the insulating film that is to bea channel protective layer; an unnecessary portion of the insulatingfilm that is to be the channel protective layer and the third oxidesemiconductor film is removed by etching; the resist mask is reduced;and an unnecessary portion of the insulating film that is to be thechannel protective layer is further removed by etching. In such amanner, the channel protective layer 406 may be formed. In this case,the resist mask which is formed first over the insulating film that isto be the channel protective layer is preferably a resist mask which isformed using a multi-tone mask and is provided with regions havingdifferent thicknesses.

Next, after the resist mask is removed, a conductive film is formed overthe n-type oxide semiconductor film.

As a material for the conductive film, an element selected fromaluminum, chromium, tantalum, titanium, molybdenum, or tungsten; analloy containing any of the above metal elements as its main component;an alloy containing the above metal elements in combination; and thelike can be given.

If heat treatment is performed after formation of the conductive film, aconductive film having at least enough heat resistance to the heattreatment is used.

Next, a photolithography step is performed. A resist mask is formed overthe conductive film, and the conductive film is etched, whereby thesource and drain electrode layers 405 are formed.

Then, a region of the n-type oxide semiconductor film, which is betweenthe source electrode and the drain electrode formed from the source anddrain electrode layers 405, is removed by etching with use of the sameresist mask, whereby the n-type oxide semiconductor layers 404 which areto be the source and drain regions are formed.

The n-type oxide semiconductor layers 404 having low resistance areprovided between the oxide semiconductor layer 403 and the source anddrain electrode layers 405, whereby the transistor 474 can operate morestably as compared with the case of using only metal wirings.

In this etching, the channel protective layer 406 functions as anetching stopper of the oxide semiconductor layer 403. Therefore, theoxide semiconductor layer 403 is not etched. The channel protectivelayer 406 is provided, so that damage to the channel formation region ofthe oxide semiconductor layer 403 (for example, reduction in filmthickness due to plasma or an etchant in etching, or oxidation) in themanufacturing process can be prevented. Therefore, the reliability ofthe transistor 474 can be improved (see FIG. 9C).

Next, the first protective insulating layer 407 is formed over thesource and drain electrode layers 405 and the channel protective layer406 (see FIG. 9D). Moisture, hydrogen ions, OH⁻, and the like arereduced in the first protective insulating layer 407, and are preventedfrom entering the first protective insulating layer 407 from theoutside. The first protective insulating layer 407 is formed using aninsulating inorganic material containing oxygen. Specifically, siliconoxide, silicon nitride, silicon oxynitride, silicon nitride oxide,aluminum oxide, aluminum nitride, magnesium oxide, yttrium oxide,hafnium oxide, or tantalum oxide can be given.

Note that steps after formation of the first protective insulating layer407 are the same as those of Embodiment 1. That is, the second gateelectrode layer 409 is formed over the first protective insulating layer407.

Note that a resin layer may be provided over the second gate electrodelayer 409. By provision of the resin layer over the second gateelectrode layer 409, unevenness due to a structure of the transistor 474can be reduced and the element can be planarized.

Note that the transistor 474 may be subjected to heat treatment under anitrogen atmosphere or an air atmosphere (in air). This heat treatmentis performed at a temperature of 300° C. or less, and the timing of theheat treatment is not particularly limited as long as it is performedafter the channel protective layer 406 is formed. For example, heattreatment is performed at 350° C. for one hour under a nitrogenatmosphere. If the heat treatment is performed, variation in electriccharacteristics of the transistor 474 can be reduced.

Through the above steps, the transistor 474 illustrated in FIG. 8A canbe formed. Note that in the transistor 474, a portion in which thechannel protective layer 406 and the first protective insulating layer407 are stacked functions as a second gate insulating layer.

A transistor 474B in FIG. 8B has a structure which is partly differentfrom that of FIG. 8A. In FIG. 8B, the same portions as those of FIG. 8Aother than different portions are denoted by the same referencenumerals.

FIG. 8B illustrates a structure in which a resin layer 408 is formedbetween the second gate electrode layer 409 and the first protectiveinsulating layer 407 which covers the transistor the first gateelectrode layer 401, the gate insulating layer 402, the oxidesemiconductor layer 403, the n-type oxide semiconductor layers 404, andthe source and drain electrode layers 405.

The resin layer 408 covers the source and drain electrode layers 405 andthe channel protective layer 406 with the first protective insulatinglayer 407 provided therebetween. The resin layer 408 can be formed usinga photosensitive or non-photosensitive organic material to have athickness of 0.5 μm to 3 μm. As the photosensitive or non-photosensitiveorganic material used for the resin layer 408, polyimide, acrylic,polyamide, polyimideamide, resist, benzocyclobutene, or a stack of anyof these materials is used. Here, a layer of photosensitive polyimide isformed by a coating method as the resin layer 408. After polyimide isapplied to the entire surface, light exposure, development, and bakingare performed, whereby the resin layer 408 of polyimide whose surface isplane and has a thickness of 1.5 μm can be formed.

By provision of the resin layer 408, unevenness due to a structure of atransistor 474B can be reduced and the surface on which the second gateelectrode layer 409 is formed can be planarized.

Note that as illustrated in FIG. 8A, the width of the second gateelectrode layer 409 is made larger than that of the first gate electrodelayer 401 and that of the oxide semiconductor layer 403, whereby gatevoltage can be applied to the entire oxide semiconductor layer 403 fromthe second gate electrode layer 409.

Note that even if the structure of FIG. 8A or FIG. 8B is employed, inthe case where a portion in which the channel protective layer 406, thefirst protective insulating layer 407, and the resin layer 408 arestacked is thin, a problem of parasitic capacitance between the secondgate electrode layer 409 and the source and drain electrode layers 405arises in some cases. In the case where a problem of parasiticcapacitance arises, the width of the second gate electrode layer 409 ismade smaller than that of the first gate electrode layer 401, and thearea where the second gate electrode layer 409 and the source and drainelectrode layers 405 overlap with each other is preferably reduced. Whenthe area where they overlap with each other is reduced, parasiticcapacitance can be reduced. Further, the width of the first gateelectrode layer 401 may be set to be smaller than that of the channelprotective layer 406 and the width of the second gate electrode 409 maybe set to be smaller than that of the channel protective layer 406 sothat the second gate electrode layer 409 does not overlap with thesource and drain electrode layers 405, whereby more parasiticcapacitance may be reduced.

Note that in the case where parasitic capacitance does not become aproblem because the portion in which the resin layer 408 and the firstprotective insulating layer 407 are stacked is sufficiently thick, thesecond gate electrode may be used as a common gate electrode whichcovers a plurality of transistors in the driver circuit and may have anarea substantially the same or larger than the area of the drivercircuit.

The channel formation region in the semiconductor layer included in thetransistor of this embodiment is a high-resistance region; thus,electric characteristics of the transistor are stabilized and increasein off current can be prevented. Therefore, a semiconductor device (adisplay device) including a transistor which has favorable electriccharacteristics and high reliability can be provided.

Note that this embodiment can be implemented in combination with any ofother embodiments described in this specification as appropriate.

Embodiment 5

In this embodiment, an example in which an inverter circuit in a drivercircuit is formed using two n-channel transistors will be described.Transistors in FIG. 10A are the same as the transistor 471 in FIG. 1A ofEmbodiment 1 or the like, and thus the same parts are denoted by thesame reference numerals. Note that n-type oxide semiconductor layers 14a and 14 b are similar to the n-type oxide semiconductor layers 404 inEmbodiment 2; a resin layer 17 is similar to the resin layer 408 inEmbodiment 1; a first protective insulating layer 18 is similar to thefirst protective insulating layer 407 in Embodiment 1; and a second gateelectrode layer 470 is similar to the second gate electrode layer 409 inEmbodiment 1.

The driver circuit for driving a pixel portion is formed using aninverter circuit, a capacitor, a resistor, and the like. When twon-channel transistors are combined to form an inverter circuit, thereare the following combinations: a combination of an enhancement typetransistor and a depletion type transistor (hereinafter, a circuitformed by such a combination is referred to as an EDMOS circuit) and acombination of enhancement type transistors (hereinafter, a circuitformed by such a combination is referred to as an EEMOS circuit).

FIG. 10A illustrates a cross-sectional structure of the inverter circuitof the driver circuit. Note that a transistor 20 and a second transistor43 in FIGS. 10A and 10B are inverted staggered channel-etchedtransistors and exemplify a transistor in which a wiring is providedover an oxide semiconductor layer with a source region or a drain regioninterposed therebetween.

In FIG. 10A, a first gate electrode 11 and a third gate electrode 42 areprovided over a substrate 10. The first gate electrode 11 and the thirdgate electrode 42 can be formed to have a single-layer structure or astacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as itsmain component.

Further, over a first gate insulating layer 13 covering the first gateelectrode 11 and the third gate electrode 42, an oxide semiconductorlayer 16 and a second oxide semiconductor layer 47 are provided.

An electrode layer serving as a first terminal (a source electrode layer15 a) and an electrode layer serving as a second terminal (a drainelectrode layer 15 b) are provided over the oxide semiconductor layer16. The electrode layer serving as the second terminal is directlyconnected to the third gate electrode 42 through a contact hole 44formed in the first gate insulating layer 13. In addition, an electrodelayer serving as a third terminal 411 is provided over the second oxidesemiconductor layer 47.

The transistor 20 includes the first gate electrode 11, the first gateinsulating layer 13 covering the first gate electrode 11, and the oxidesemiconductor layer 16 overlapping with the first gate electrode 11 withthe first gate insulating layer 13 between the first gate electrode 11and the oxide semiconductor layer 16. The electrode layer serving as thefirst terminal (the source electrode layer 15 a) is a power supply lineto which negative voltage VDL is applied (a negative power supply line).This power supply line may be a power supply line with a groundpotential (a ground potential power supply line). Note that in theinverter circuit, the electrode layer serving as the first terminal isthe drain electrode layer and the electrode layer serving as the secondterminal is the source electrode layer in some cases, depending on apotential of a wiring connected to the electrode layer serving as thesecond terminal (the drain electrode layer 15 b).

The second transistor 43 includes the third gate electrode 42 and thesecond oxide semiconductor layer 47 overlapping with the third gateelectrode 42 with the first gate insulating layer 13 between the thirdgate electrode 42 and the second oxide semiconductor layer 47. The thirdterminal 411 is a power supply line to which positive voltage VDH isapplied (a positive power supply line). Note that in the invertercircuit, the electrode layer serving as the second terminal is thesource electrode layer and the electrode layer serving as the thirdterminal 411 is the drain electrode layer in some cases, depending on apotential of a wiring connected to the electrode layer serving as thesecond terminal (the drain electrode layer 15 b).

Here, a buffer layer 408 a (also referred to as a source region or adrain region) is provided between the second oxide semiconductor layer47 and the drain electrode layer 15 b. A buffer layer 408 b (alsoreferred to as a drain region or a source region) is provided betweenthe second oxide semiconductor layer 47 and the third terminal 411.

FIG. 10B is a top view of the inverter circuit of the driver circuit. InFIG. 10B, a cross section taken along chain line Z1-Z2 corresponds toFIG. 10A.

In this embodiment, in order that the transistor 20 can serve as ann-channel enhancement type transistor, a second gate insulating layer isprovided over the oxide semiconductor layer 16 and a second gateelectrode 19 is provided over the second gate insulating layer so thatthe threshold voltage of the transistor 20 is controlled by voltageapplied to the second gate electrode 19.

Note that the example in which the electrode layer serving as the secondterminal (the drain electrode layer 15 b) is directly connected to thethird gate electrode 42 through the contact hole 44 formed in the firstgate insulating layer 13 is illustrated in FIGS. 10A and 10B withoutparticular limitations. The electrode layer serving as the secondterminal (the drain electrode layer 15 b) may be electrically connectedto the third gate electrode 42 with a connection electrode separatelyprovided.

This embodiment can be implemented in combination with any ofEmbodiments 1 to 4 as appropriate.

Embodiment 6

In this embodiment, a display device which is one embodiment of thepresent invention will be described with reference to a block diagram, acircuit diagram, a waveform diagram showing potential changes ofsignals, a top view (a layout diagram), and the like.

An example of a block diagram of an active matrix liquid crystal displaydevice is illustrated in FIG. 11A. The liquid crystal display deviceillustrated in FIG. 11A includes, over a substrate 800, a pixel portion801 including a plurality of pixels each provided with a displayelement, a scan line driver circuit 802 which controls potentials ofscan lines connected to gate electrodes of the pixels, and a signal linedriver circuit 803 which controls a video signal input to a selectedpixel. Each pixel is provided with a transistor 804 in FIG. 11B. Thetransistor 804 is an element controlling electric current between an Interminal and an Out terminal with a first control signal G1 and a secondcontrol signal G2. Note that a symbol of the transistor 804 in FIG. 11Bcorresponds to the transistor described in any one of Embodiments 1 to4.

Note that although a mode in which the scan line driver circuit 802 andthe signal line driver circuit 803 are formed over the substrate 800 isdescribed here, part of the scan line driver circuit 802 may be mountedover an IC formed over another substrate. Further, part of the signalline driver circuit 803 may be mounted over an IC formed over anothersubstrate. Still further, a plurality of scan line driver circuits 802may be provided over the substrate 800.

FIG. 12 illustrates a positional relationship of signal input terminals,scan lines, signal lines, protective circuits including non-linearelements, and a pixel portion in a display device. Over a substrate 820having an insulating surface, scan lines 823A and control lines 823Bintersect with signal lines 824 in a pixel portion 827. The pixelportion 827 corresponds to the pixel portion 801 in FIG. 11. Note thatthe control lines 823B may be arranged parallel to the signal line 824.

The pixel portion 827 includes a plurality of pixels 828 arranged in amatrix. The pixel 828 includes a pixel transistor 829 connected to thescan line 823A, the control line 823B, and the signal line 824, astorage capacitor 830, and a pixel electrode 831.

The pixel structure here illustrates a case where one electrode of thestorage capacitor 830 is connected to the pixel transistor 829 and theother electrode of the storage capacitor 830 is connected to a capacitorline 832. The pixel electrode 831 serves as one of electrodes whichdrive a display element (such as a liquid crystal element, alight-emitting element, or a contrast medium (electronic ink)). Theother electrode (also referred to as a counter electrode) of the displayelement is connected to a common terminal 833. From the common terminal,a common potential is applied to the counter electrode of the displayelement.

The protective circuit 835 is provided between a wiring extended fromthe pixel portion 827 and the signal line input terminal 822. Theprotective circuit 835 is also provided between the scan line drivercircuit 802 and the pixel portion 827. In this embodiment, theprotective circuit 835 including a plurality of protective circuits isprovided so that the pixel transistors 829 and the like are not brokenwhen surge voltage due to static electricity or the like is applied tothe scan line 823A, the control line 823B, the signal line 824, or thecapacitor line 832. Accordingly, the protective circuits 835 are formedso that charge can be released into a common wiring when surge voltageis applied.

In this embodiment, an example in which one protective circuit isprovided for each wiring in the vicinity of the signal line inputterminals 822 is shown. However, the position of the protective circuits835 and the number of protective circuits provided in the protectivecircuit 835 are not limited to the example.

The use of the transistor described in any of Embodiments 1 to 4 as thepixel transistor 829 allows the threshold voltage of the pixeltransistor 829 to be controlled and/or on current of the transistor tobe increased.

FIG. 13A is a waveform diagram schematically showing potential changesof signals supplied to the pixel 828. Here, operation of the pixel 828will be described. FIG. 13A shows a waveform of potentials of each ofthe scan line 823A, the control line 823B, the signal line 824, and thecapacitor line 832 which are connected to one pixel. In FIG. 13A, awaveform G1 schematically represents a potential change of the scan line823A, a waveform G2 schematically represents a potential change of thecontrol line 823B, a waveform D schematically represents a potentialchange of the signal line 824, and a waveform COM schematicallyrepresents a potential change of the capacitor line 832. Changes inthose waveforms over time are shown with the horizontal axisrepresenting time and the vertical axis representing potential. Notethat a high power supply potential of the waveform G1 is denoted as V₁and a low power supply potential of the waveform G1 is denoted as V₂. Apotential of the waveform G2 is denoted as V_(c). A high power supplypotential of the waveform D is denoted as V_(D1) and a low power supplypotential of the waveform D is denoted as V_(D2). A potential of thewaveform COM is denoted as V_(COM). As shown in FIGS. 13A and 13B, aperiod of time from when the waveform G1 changes to V₁, until thewaveform G1 changes to V₁ again after changing to V₂ corresponds to oneframe period. Further, as shown in FIGS. 13A and 13B, a period of timefrom when the waveform G1 changes to V₁ until the waveform G1 changes toV₂ corresponds to one gate selection period.

In FIG. 13A, in one gate selection period in one frame period, that is,in a period of time when the scan line 823A has V₁, the storagecapacitor 830 in the pixel 828 holds a potential of the signal line 824in the range of from V_(D1) to V_(D2). In FIG. 13A, a period other thana gate selection period in one frame period, that is, in a period oftime when the scan line 823A has V₂, the storage capacitor 830 in thepixel 828 holds a potential input in one gate selection periodregardless of the potential of the signal line 824, which is in therange of from V_(D1) to V_(D2). Note that the waveform G2 schematicallyrepresenting a potential change of the control line 823B is preferablykept at a fixed potential in the range in which the 823B does not causemalfunction of the pixel transistor 829 which is controlled on or off bythe scan line 823A. By setting the potential V_(c) of the control line823B at V_(D2) or lower, preferably in the range of from V₂ to V_(D2),malfunction of the pixel transistor 829 which is controlled on or off bythe scan line 823A can be prevented.

FIG. 13B is another example of a waveform diagram schematically showingpotential changes in the case where a potential of the signal line 824is fixed at V_(D1) for a certain period of time. FIG. 13B is differentfrom FIG. 13A in that the waveform D representing a potential change ofthe signal line 824 is specifically shown (in FIG. 13A, the waveform Drepresents a given potential in the range of from V_(D2) to V_(D1)), andthat a waveform C_(pix) representing a change of a potential held by thestorage capacitor 830 in the pixel 828 is shown. In FIG. 13B, before thewaveform G1 changes to V₁, the waveform D changes to V_(D1) from V_(D2),and then the waveform G1 changes to V₁ and a potential held by thestorage capacitor 830 in the pixel 828, that is, a potential of thewaveform C_(pix) rises (see the first one gate selection period in FIG.13B). In addition, in FIG. 13B, before the waveform G1 changes to V₁,the waveform D changes to V_(D2) from V_(D1), and then the waveform G1changes to V₁ and a potential of the storage capacitor 830 in the pixel828, that is, a potential of the waveform C_(pix) falls (see the secondone gate selection period in FIG. 13B). If the waveform D changes toV_(D1) from V_(D2) or V_(D2) from V_(D1) before the waveform G1 changesto V₁, malfunction due to signal delay and the like can be reduced. Notethat in FIG. 13B, although there is a period in which the waveform D andthe waveform C_(pix) are in the same potential, they are separatelyshown for the sake of clarity.

As shown in FIGS. 13A and 13B, by provision of the control line 823B,the threshold voltage of the pixel transistor 829 can be controlledwhile a similar effect of the transistor described in any one ofEmbodiments 1 to 4 is obtained. Specifically, by setting a potential ofthe waveform G2 of the control line 823B at a fixed potential, atransistor with a stable threshold voltage can be obtained, which ispreferable.

Note that the waveform diagrams in FIGS. 13A and 13B schematicallyshowing potential changes of signals supplied to the pixel 828 aremerely examples and may be combined with another driving method. As anexample of another driving method, a driving method such as an inversiondrive method (so-called inversion drive) may be employed, in which thepolarity of a voltage applied to the pixel electrode is inverted everycertain period or every frame or between pixels in accordance with thecommon potential of the common electrode. By the inversion drive, unevendisplay such as flickering of an image and deterioration of a displayelement (e.g., a liquid crystal material) can be suppressed. Note thatas an example of the inversion drive, source line inversion drive, gateline inversion drive, dot inversion drive, and the like can be given aswell as frame inversion drive. Note that as a display method, aprogressive method, an interlace method, or the like can be employed.Further, one pixel may include a plurality of subpixels.

FIG. 14 is an example of a layout diagram of the pixel 828 in FIG. 12.FIG. 14 shows an example where a structure of a transistor is achannel-etch type described in Embodiment 1. In FIG. 14, a cross sectiontaken along chain line A-B corresponds to the cross-sectional view ofFIG. 1C. Note that the layout diagram of pixels of FIG. 14 shows anexample of so-called stripe arrangement in which pixels of three colors,RGB (R is red, G is green, and B is blue), are arranged along the scanline 823A; however, the arrangement is not limited thereto, and delta orBayer arrangement may alternatively be employed. Note that withoutlimitation to the three colors of RGB, more than three colors may beused. For example, RGBW (W is white) or RGB with one or more colors ofyellow, cyan, or magenta may be used. Note that areas of display regionsin pixels may be different between color elements of RGB.

FIG. 14 illustrates a pixel circuit including a first conductive layer1101 which serves as a wiring serving as the scan line 823A and oneelectrode of the capacitor line 832, an oxide semiconductor layer 1102which forms a channel region of the pixel transistor 829, a secondconductive layer 1103 which serves as a wiring serving as the signalline 824 and the other electrode of the capacitor line 832, a pixelelectrode layer 1104 which serves as the pixel electrode 831, a thirdconductive layer 1105 which serves as a wiring serving as the controlline 823B, and an opening 1106 (referred to as a contact hole) forconnection between the second conductive layer 1103 and the pixelelectrode 831. Although FIG. 14 shows a structure in which the thirdconductive layer 1105 parallel to the first conductive layer 1101 isextended over the oxide semiconductor layer 1102, a structure in FIG. 15in which the third conductive layer 1105 is provided to overlap with thefirst conductive layer 1101 and the oxide semiconductor layer 1102 maybe employed. When the third conductive layer 1105 is formed from alight-blocking conductive material, the light-blocking property of thethird conductive layer 1105 can be more improved in the structure inFIG. 15, than that in the layout diagram in FIG. 14.

Note that, in the layout diagram of FIG. 14 or the like, the facingportion of source and drain regions in the transistor may have a U-likeshape or a C-like shape. Further, the first conductive layer 1101serving as a first gate electrode may have a U-like shape or a C-likeshape. Note that the width in the channel length direction of the firstconductive layer 1101 which serves as the first gate electrode may belarger than the width of the oxide semiconductor layer 1102. Inaddition, the width in a channel length direction of the thirdconductive layer 1105 which serves as the second gate electrode issmaller than the width of the first conductive layer 1101 and the widthof the oxide semiconductor layer 1102.

FIG. 16 illustrates an example in which connection between the pixeltransistors and the scan lines is different from that in FIG. 12. FIG.16 illustrates the case where the first gate electrode connected to thescan line and the second gate electrode connected to the control lineare connected to each other and have the same potential with use of thetransistor described in any one of Embodiments 1 to 4. Note that thesame portions in FIG. 16 as those in FIG. 12 are not repeatedlydescribed.

FIG. 16 illustrates a positional relationship of signal input terminals,scan lines, signal lines, protective circuits including non-linearelements, and a pixel portion in a display device. FIG. 16 is differentfrom FIG. 12 in that the control line 823B is not provided and the scanline 823 which corresponds to the scan line 823A in FIG. 12 is provided.As shown in FIG. 16, by controlling the pixel transistors with the scanline 823 connected to the second gate electrode, the control line can beomitted, which can decrease the number of wirings and signal line inputterminals 822.

FIG. 17 is a waveform diagram schematically showing potential changes ofsignals supplied to the pixel 828 shown in FIG. 16. Here, operation ofthe pixel 828 in FIG. 16 will be described. FIG. 17 shows a waveform ofpotentials of each of the scan line 823, the signal line 824, and thecapacitor line 832 which are connected to one pixel. Note that in FIG.17, in order to clarify the difference from FIG. 13A, the first gateelectrode and the second gate electrode which are connected to the scanline 823 such that they have the same potential are shown to beseparated slightly from each other. In FIG. 17, a waveform G1schematically represents a potential change of the first gate electrode,a waveform G2 schematically represents a potential change of the secondgate electrode, a waveform D schematically represents a potential changeof the signal line 824, and a waveform COM schematically represents apotential change of the capacitor line 832. Changes in those waveformsover time are shown with the horizontal axis representing time and thevertical axis representing potential. Note that a high power supplypotential of the waveform G1 and the waveform G2 is denoted as V₁ and alow power supply potential of the waveform G1 and the waveform G2 isdenoted as V₂. A high power supply potential of the waveform D isdenoted as V_(D1) and a low power supply potential of the waveform D isdenoted as V_(D2). A potential of the waveform COM is denoted asV_(COM). As shown in FIG. 17, a period of time from when the waveform G1changes to V₁, until the waveform G1 changes to V₁ again after changingto V₂ corresponds to one frame period. Further, as shown in FIG. 17, aperiod of time from when the waveform G1 changes to V₁ until thewaveform G1 changes to V₂ corresponds to one gate selection period.

In FIG. 17, in one gate selection period in one frame period, that is,in a period of time when the scan line 823 has V₁, the storage capacitor830 in the pixel 828 holds a potential of the signal line 824 in therange of from V_(D1) to V_(D2). In FIG. 17, a period other than a gateselection period in one frame period, that is, in a period of time whenthe scan line 823 has V₂, the storage capacitor 830 in the pixel 828holds a potential input in one gate selection period regardless of thepotential of the signal line 824, which is in the range of from V_(D1)to V_(D2).

By driving the pixel transistor 829 in a manner in which the waveform G1and the waveform G2 are in the same potential as shown in FIG. 17, anarea which becomes a channel in the pixel transistor 829 can beincreased. Thus, an amount of current flowing through the pixeltransistor 829 is increased, whereby the display element can operate athigh speed. As another structure in which the pixel transistor 829 isdriven in a manner in which the waveform G1 and the waveform G2 are inthe same potential, a structure provided with a first scan line drivercircuit 802A and a second scan line driver circuit 802B shown in FIG. 18can be given. As shown in FIG. 18, the transistor may be controlled bythe first scan line driver circuit 802A and the second scan line drivercircuit 802B which supply scan signals through the first scan line 823Cand the second scan line 823D, respectively.

Note that the waveform diagram in FIG. 17 schematically showingpotential changes is one example similarly to the waveform diagrams inFIGS. 13A and 13B and may be combined with another driving method. As anexample of another driving method, the driving method (so-calledinversion drive described above) may be employed, in which the polarityof a voltage applied to the pixel electrode is inverted every certainperiod or every frame or between pixels in accordance with the commonpotential of the common electrode. With use of the inversion drive,effects similar to the above can be obtained.

FIG. 19 is an example of a layout diagram of the pixel 828 in FIG. 16.FIG. 19 shows an example where a structure of a transistor is achannel-etch type described in Embodiment 1. Note that the layoutdiagram of pixels of FIG. 19 shows an example of so-called stripearrangement in which pixels of three colors, RGB (R is red, G is green,and B is blue), are arranged along the scan line 823; however, thearrangement is not limited thereto, and delta or Bayer arrangement mayalternatively be employed. Note that without limitation to the threecolors of RGB, more than three colors may be used. For example, RGBW (Wis white) or RGB with one or more colors of yellow, cyan, or magenta maybe used. Note that areas of display regions in pixels may be differentbetween color elements of RGB.

FIG. 19 illustrates a pixel circuit including a first conductive layer1101 which serves as a wiring serving as the scan line 823 and oneelectrode of the capacitor line 832, an oxide semiconductor layer 1102which forms a channel region of the pixel transistor 829, a secondconductive layer 1103 which serves as a wiring serving as the signalline 824 and the other electrode of the capacitor line 832, a pixelelectrode layer 1104 which serves as the pixel electrode 831, a thirdconductive layer 1105 which is connected to the first conductive layer1101, and an opening 1106 (referred to as a contact hole) for connectionbetween the second conductive layer 1103 and the pixel electrode 831 orbetween the first conductive layer 1101 and the third conductive layer1105. Although FIG. 19 shows a structure in which the third conductivelayer 1105 is provided over the oxide semiconductor layer 1102 for eachtransistor 829, a structure in FIG. 20 in which the third conductivelayer 1105 is provided to overlap with the first conductive layer 1101and the oxide semiconductor layer 1102 may be employed. When the thirdconductive layer 1105 is formed from a light-blocking conductivematerial, the light-blocking property of the third conductive layer 1105can be more improved in the structure in FIG. 20, than that in thelayout diagram in FIG. 19.

Note that, in the layout diagram of FIG. 19 or the like, the facingportion of source and drain regions in the transistor may have a U-likeshape or a C-like shape. Further, the first conductive layer 1101serving as a gate electrode may have a U-like shape or a C-like shape.Note that the width in the channel length direction of the firstconductive layer 1101 which serves as the first gate electrode may belarger than the width of the oxide semiconductor layer 1102. Inaddition, the width in a channel length direction of the thirdconductive layer 1105 which serves as the second gate electrode islarger than the width of the first conductive layer 1101 and the widthof the oxide semiconductor layer 1102.

As described above, by use of the transistor having the structuredescribed in any one of Embodiments 1 to 4, the threshold voltage can becontrolled to a favorable value while effects described in the aboveembodiments can be obtained.

Note that in this embodiment, what is illustrated in the drawing can befreely combined with or replaced with what is described in anotherembodiment as appropriate.

Embodiment 7

In this embodiment, a light-emitting display device to which thetransistor including an oxide semiconductor layer described in any oneof Embodiments 1 to 4 is applied will be described. Note that as anexample of a display element included in the light-emitting displaydevice of this embodiment, a light-emitting element utilizingelectroluminescence is described. Light-emitting elements utilizingelectroluminescence are classified according to whether a light emittingmaterial is an organic compound or an inorganic compound. The former isreferred to as an organic EL element and the latter is referred to as aninorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions.

Note that description is made in this embodiment using an organic ELelement as a light-emitting element.

FIG. 21 shows an example of a pixel in a light-emitting display deviceincluding the transistor described in any one of Embodiments 1 to 4.

A structure and an operation of the pixel in the light-emitting displaydevice are described. In this example, one pixel includes two n-channeltransistors each of which includes an oxide semiconductor layer (forexample, an In—Ga—Zn—O-based non-single-crystal film) as a channelformation region.

A pixel 6400 includes a switching transistor 6401 (a first transistor),a driver transistor 6402 (a second transistor), a capacitor 6403, and alight-emitting element 6404. The switching transistor 6401 has a firstgate electrode connected to a scan line 6406A, a second gate electrodeconnected to a control line 6406B, a first electrode (one of a sourceelectrode and a drain electrode) connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) connected to a gate of the driver transistor 6402. The drivertransistor 6402 has a first gate electrode connected to a power supplyline 6407 through the capacitor 6403, a second gate electrode connectedto the control line 6406B, a first electrode connected to the powersupply line 6407, and a second electrode connected to a first electrode(a pixel electrode) of the light-emitting element 6404. A secondelectrode of the light-emitting element 6404 corresponds to a commonelectrode 6408. The common electrode 6408 is electrically connected to acommon potential line provided over the same substrate, and theconnection portion may be used as a common connection portion.

Note that the second electrode (the common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Thelow power supply potential is a potential smaller than a high powersupply potential when the high power supply potential set to the powersupply line 6407 is a reference. As the low power supply potential, GND,0 V, or the like may be employed, for example. A potential differencebetween the high power supply potential and the low power supplypotential is applied to the light-emitting element 6404 to make currentflow through the light-emitting element 6404, so that the light-emittingelement 6404 emits light. Thus, each of the potentials is set so thatthe potential difference between the high power supply potential and thelow power supply potential is equal to or higher than the forwardthreshold voltage of the light-emitting element 6404.

Note that gate capacitance of the driver transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel region and the gate electrode, for example.

In the case of analog grayscale driving, voltage equal to or higher thanthe sum of the forward voltage of the light-emitting element 6404 andthe threshold voltage of the driver transistor 6402 is applied to thefirst gate of the driver transistor 6402. The forward voltage of thelight-emitting element 6404 indicates a voltage at which a desiredluminance is obtained, and includes at least forward threshold voltage.The video signal by which the driver transistor 6402 operates in asaturation region is input, so that current can be supplied to thelight-emitting element 6404. In order to allow the driver transistor6402 to operate in the saturation region, the potential of the powersupply line 6407 is set higher than the potential of the first gate ofthe driver transistor 6402. When an analog video signal is used, currentcan be made to flow through the light-emitting element 6404 inaccordance with the video signal and analog grayscale driving can beperformed.

As shown in FIG. 21, by provision of the control line 6406B, thethreshold voltage of the switching transistor 6401 and the drivertransistor 6402 can be controlled as in the transistor described in anyone of Embodiments 1 to 4. Specifically, in the driver transistor 6402,a video signal is input so that the driver transistor 6402 operates inthe saturation region. Therefore, by controlling the threshold voltageby a potential of the control line 6406B, a deviation between an inputvideo signal and luminance of the light-emitting element due tothreshold voltage shift can be reduced. As a result, display quality ofthe display device can be improved.

Note that the switching transistor 6401 serves as a switch and apotential of the second gate is not always required to be controlled bythe control line 6406B. That is, the control line 6406B may be connectedto only the second gate of the driver transistor 6402.

Note that the pixel structure illustrated in FIG. 21 is not limitedthereto. For example, a switch, a resistor, a capacitor, a transistor, alogic circuit, or the like may be added to the pixel in FIG. 21.

In the case of digital grayscale driving, a video signal is input to thegate of the driver transistor 6402 so that the driver transistor 6402 iseither completely turned on or completely turned off. That is, thedriver transistor 6402 operates in a linear region. Since the drivertransistor 6402 operates in a linear region, the potential of the firstgate of the driver transistor 6402 is set higher than the potential ofthe power supply line 6407. Note that voltage which is equal to orhigher than the sum of the voltage of the power supply line and the Vthof the driver transistor 6402 is applied to the signal line 6405. Inthis case, the same structure as in FIG. 21 can be employed.

Next, structures of a light-emitting element will be described withreference to FIGS. 22A to 22C. A cross-sectional structure of a pixel isdescribed here by taking an n-channel driver transistor as an example.Transistors 7001, 7011, and 7021 serving as driver transistorsillustrated in FIGS. 22A to 22C can be formed by a method similar to themethod for forming the transistor 471 described in Embodiment 1 or thelike. The transistors 7001, 7011, and 7021 each include an oxidesemiconductor layer for a channel formation region.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode should be transparent. There are thefollowing structures of a light-emitting element which is formed overthe same substrate as a transistor: a top-emission structure in whichlight is extracted through the surface opposite to the substrate, abottom-emission structure in which light is extracted through thesurface of the substrate, and a dual-emission structure in which lightis extracted through the surface opposite to the substrate and thesurface of the substrate. As illustrated in FIGS. 22A to 22C, any ofthese emission structures can be applied in this embodiment.

A light-emitting element having a top-emission structure will bedescribed with reference to FIG. 22A.

FIG. 22A is a cross-sectional view of a pixel in which the transistor7001 described in Embodiment 1 is provided as a driver transistor in thepixel and light emitted from a light-emitting element 7002 electricallyconnected to the transistor 7001 goes out through an anode 7005. Thetransistor 7001 is covered with a protective layer 7007 and a resinlayer 7017 over which a second protective insulating layer 7018 formedof a silicon nitride film is provided. An In—Zn—O-based oxidesemiconductor is used for the channel of the transistor 7001.

In FIG. 22A, a cathode 7003 of the light-emitting element 7002 iselectrically connected to the transistor 7001 serving as a drivertransistor, and a light-emitting layer 7004 and the anode 7005 arestacked in this order over the cathode 7003. The cathode 7003 can beformed using any of conductive materials which have a low work functionand a film of which reflects light. For example, Ca, Al, MgAg, AlLi, orthe like is preferably used.

In FIG. 22A, a second gate electrode 7009 which is formed from the samematerial as the cathode 7003 overlaps with the oxide semiconductor layerto shield the oxide semiconductor layer from light. In addition, thesecond gate electrode 7009 controls the threshold voltage of thetransistor 7001. By formation of the cathode 7003 and the second gateelectrode 7009 from the same material and the same layer, the number ofsteps can be reduced.

In addition, a partition 7006 formed of an insulating material isprovided in order to prevent short circuit of the second gate electrode7009 and the cathode 7003. The light-emitting layer 7004 is provided soas to overlap with both of part of the partition 7006 and part of thecathode 7003 which is not covered with the partition 7006.

The light-emitting layer 7004 may be formed using either a single layeror a stacked layer of a plurality of layers. When the light-emittinglayer 7004 is formed using a stacked layer of a plurality of layers, anelectron-injection layer, an electron-transport layer, a light-emittinglayer, a hole-transport layer, and a hole-injection layer aresequentially stacked over the cathode 7003. It is not necessary to formall of these layers. The anode 7005 is formed using a light-transmittingconductive material such as a film of indium oxide including tungstenoxide, indium zinc oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide including titanium oxide,indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added.

The light-emitting element 7002 includes the cathode 7003, the anode7005, and the light-emitting layer 7004 sandwiched between the cathode7003 and the anode 7005. In the case of the pixel illustrated in FIG.22A, light is emitted from the light-emitting element 7002 to the anode7005 side as indicated by an arrow.

Next, a light-emitting element having a bottom-emission structure willbe described with reference to FIG. 22B.

FIG. 22B is a cross-sectional view of a pixel in which the transistor7011 described in Embodiment 1 is provided as a driver transistor in thepixel and light emitted from a light-emitting element 7012 electricallyconnected to the transistor 7011 goes out through a cathode 7013. Thetransistor 7011 is covered with the protective layer 7007 and the resinlayer 7017 over which the second protective insulating layer 7018 formedof a silicon nitride film is provided. An In—Ga—Zn—O-based oxidesemiconductor is used for the channel of the transistor 7011.

In FIG. 22B, the cathode 7013 of the light-emitting element 7012 isformed over a conductive film 7010 having a light-transmitting propertywhich is electrically connected to the transistor 7011 which is thedriver transistor, and a light-emitting layer 7014 and an anode 7015 arestacked in this order over the cathode 7013. Note that a blocking film7016 for reflecting or blocking light may be formed so as to cover theanode 7015 when the anode 7015 has a light-transmitting property. Forthe cathode 7013, any of conductive materials which have a low workfunction can be used as in the case of FIG. 22A. Note that the cathode7013 is formed to have a thickness with which the cathode 7013 transmitslight (preferably, approximately from 5 nm to 30 nm). For example, analuminum film with a thickness of 20 nm can be used as the cathode 7013.

Similarly to the case of FIG. 22A, the light-emitting layer 7014 may beformed using either a single layer or a stacked layer of a plurality oflayers. The anode 7015 is not required to transmit light, but can beformed using a light-transmitting conductive material as in the case ofFIG. 22A. The blocking film 7016 can be formed using, for example, ametal which reflects light; however, it is not limited to a metal film.For example, a resin to which a black pigment is added can be used.

In FIG. 22B, a second gate electrode 7019 which is formed from the samelight-transmitting conductive material as the conductive film 7010having a light-transmitting property overlaps with the oxidesemiconductor layer. In this embodiment, indium tin oxide includingsilicon oxide is used as a material for the second gate electrode 7019.The second gate electrode 7019 controls the threshold voltage of thetransistor 7011. By formation of the conductive film 7010 having alight-transmitting property and the second gate electrode 7019 from thesame material and the same layer, the number of steps can be reduced.The oxide semiconductor layer in the transistor 7011 is shielded fromlight by the blocking film 7016 provided over the second gate electrode7019.

The light-emitting element 7012 includes the cathode 7013, the anode7015, and the light-emitting layer 7014 sandwiched between the cathode7013 and the anode 7015. In the case of the pixel illustrated in FIG.22B, light is emitted from the light-emitting element 7012 to thecathode 7013 side as indicated by an arrow.

Next, a light-emitting element having a dual-emission structure will bedescribed with reference to FIG. 22C.

FIG. 22C is a cross-sectional view of a pixel in which the transistor7021 described in Embodiment 1 is provided as a driver transistor in thepixel and light emitted from a light-emitting element 7022 electricallyconnected to the transistor 7021 goes out through both of an anode 7025and a cathode 7023. The transistor 7021 is covered with the protectivelayer 7007 and the resin layer 7017 over which the second protectiveinsulating layer 7018 formed of a silicon nitride film is provided. AZn—O-based oxide semiconductor is used for the channel of the transistor7021.

The cathode 7023 of the light-emitting element 7022 is formed over aconductive film 7027 having a light-transmitting property which iselectrically connected to the transistor 7021 via a connection electrode7028, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. For the cathode 7023, any ofconductive materials which have a low work function can be used as inthe case of FIG. 22A. Note that the cathode 7023 is formed to have athickness with which the cathode 7023 transmits light (preferably,approximately from 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm can be used as the cathode 7023.

Similarly to the case of FIG. 22A, the light-emitting layer 7024 may beformed using either a single layer or a stacked layer of a plurality oflayers. The anode 7025 can be formed using a light-transmittingconductive material as in the case of FIG. 22A.

The light-emitting element 7022 includes the cathode 7023, the anode7025, and the light-emitting layer 7024 sandwiched between the cathode7023 and the anode 7025. In the case of the pixel illustrated in FIG.22C, light is emitted from the light-emitting element 7022 to both theanode 7025 side and the cathode 7023 side as indicated by arrows.

In FIG. 22C, a second gate electrode 7029 overlaps with the oxidesemiconductor layer. Therefore, as a material for the second gateelectrode 7029, a light-blocking conductive material (such as Ti,titanium nitride, Al, or W) is used. Here, titanium is used as amaterial for the second gate electrode 7029. The second gate electrode7029 controls the threshold voltage of the transistor 7021. The oxidesemiconductor layer in the transistor 7021 is shielded from light by thesecond gate electrode 7029. The second gate electrode 7029 and theconnection electrode 7028 which is connected to the transistor 7021 areformed from the same material (that is, titanium) and the same layer.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element may be used as a light-emittingelement.

Note that although the example in which a transistor (a drivertransistor) which controls driving of a light-emitting element isconnected to the light-emitting element is described in this embodiment,a transistor for controlling current may be connected between the drivertransistor and the light-emitting element.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which is oneembodiment of the semiconductor device of the present invention, will bedescribed with reference to FIGS. 23A and 23B. FIG. 23A is a top view ofa light-emitting display panel in which a transistor and alight-emitting element formed over a first substrate are sealed betweenthe first substrate and a second substrate with a sealant. FIG. 23B is across-sectional view taken along line H-I of FIG. 23A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4500. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4503, by the first substrate 4500, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air as described above.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4500 each include a plurality of transistors, and atransistor 4510 included in the pixel portion 4502 and a transistor 4509included in the signal line driver circuit 4503 a are illustrated as anexample in FIG. 23B.

Here, the transistors 4509 and 4510 include a Zn—O-based oxidesemiconductor. In this embodiment, the transistors 4509 and 4510 aren-channel transistors. The transistors 4509 and 4510 are covered with aresin layer 4508 which is provided over the first protective layer 4507,and a second protective insulating layer 4514 which is provided over theresin layer 4508. Note that the second protective insulating layer 4514formed using silicon nitride is formed to cover a top surface and sidesurfaces of the resin layer 4508. A second gate electrode 4522 isprovided as a top layer of the transistor 4509, and a second gateelectrode 4521 is provided as a top layer of the transistor 4510. Thesecond gate electrodes 4521 and 4522 are formed from the same layer, andthey each control the threshold voltage of the transistor, and functionas a protective layer for the oxide semiconductor layer.

The width of the second gate electrode 4522 may be larger than that ofthe gate electrode of the transistor 4509 so that gate voltage can beapplied to the entire oxide semiconductor layer. In the case where thesecond gate electrode 4522 is formed using a light-blocking conductivematerial, the oxide semiconductor layer of the transistor 4509 can beshielded from light. In the case where the second gate electrode 4522 isformed using a light-blocking conductive material, changes in electriccharacteristics of the transistor due to photosensitivity of the oxidesemiconductor can be prevented and thus the transistor can operatestably.

The width of the second gate electrode 4521 is different from that ofthe second gate electrode 4522 and is smaller than that of the firstgate electrode of the transistor 4510. When the width of the second gateelectrode 4521 is made smaller than that of the first gate electrode ofthe transistor 4510, an area in which the second gate electrode 4521overlaps with the source electrode or the drain electrode of thetransistor 4510 is reduced, whereby parasitic capacitance can bereduced. The width of the second gate electrode 4521 is smaller thanthat of the oxide semiconductor layer of the transistor 4510; thus, thesecond gate electrode 4521 shields only part of the oxide semiconductorlayer from light, but a second electrode layer 4513 is provided over thesecond gate electrode 4521. When the second electrode layer 4513 isformed using a light-blocking conductive material, the entire part ofthe oxide semiconductor layer can be shielded from light.

A first electrode layer 4517 that is a pixel electrode included in thelight-emitting element 4511 is connected to a source electrode or adrain electrode of the transistor 4510. Note that the light-emittingelement 4511 has a structure in which the first electrode layer 4517, anelectroluminescent layer 4512, and the second electrode layer 4513 arestacked, but it is not limited to the structure. The structure of thelight-emitting element 4511 can be changed as appropriate depending onthe direction in which light is extracted from the light-emittingelement 4511, or the like.

A partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed using a photosensitivematerial and an opening be formed over the first electrode layer 4517 sothat a sidewall of the opening is formed as an inclined surface withcontinuous curvature.

The electroluminescent layer 4512 may be formed using either a singlelayer or a stacked layer of a plurality of layers.

A protective film may be formed over the second electrode layer 4513 andthe partition wall 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

A variety of signals and potentials are supplied to the signal linedriver circuits 4503 a and 4503 b, the scan line driver circuits 4504 aand 4504 b, or the pixel portion 4502 from FPCs 4518 a and 4518 b.

In this embodiment, a connection terminal electrode 4515 and the firstelectrode layer 4517 which is included in the light-emitting element4511 are formed from the same material and the same layer. A terminalelectrode 4516 and the source and drain electrodes which are included inthe transistors 4509 and 4510 are formed from the same material and thesame layer. Note that a gate insulating layer 4501 of the transistors4509 and 4510 is provided below the terminal electrode 4516

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmittingsubstrate such as a glass plate, a plastic plate, a polyester film, oran acrylic film is used.

As the filler 4503, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. Here, nitrogen is used for the filler 4503.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be provided using a single crystalsemiconductor film or polycrystalline semiconductor film over anothersubstrate. In addition, only the signal line driver circuits, or onlythe scan line driver circuits or part thereof may be separately formedover another substrate.

Through the above steps, a highly reliable light-emitting display device(display panel) as a semiconductor device can be manufactured.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 8

In this embodiment, a liquid crystal display device to which thetransistor including an oxide semiconductor layer described in any oneof Embodiments 1 to 4 is applied will be described. A liquid crystaldisplay device having a display function can be manufactured using thetransistors including an oxide semiconductor layer which are describedin any one of Embodiments 1 to 4 not only in a driver circuit but alsoin a pixel portion. Further, part or whole of a driver circuit can beformed over the same substrate as a pixel portion, using the transistor,whereby a system-on-panel can be obtained.

The liquid crystal display device includes a liquid crystal element (aliquid crystal display element) as a display element.

In addition, the liquid crystal display device includes a panel in whicha display element is sealed, and a module in which an IC and the likeincluding a controller are mounted on the panel. The liquid crystaldisplay device also includes one mode of an element substrate before thedisplay element is completed in a manufacturing process of the liquidcrystal display device, and the element substrate is provided with ameans to supply current to the display element in each pixel.Specifically, the element substrate may be in a state after only a pixelelectrode of the display element is formed, a state after a conductivefilm to be a pixel electrode is formed but before the conductive film isetched to be the pixel electrode, or any other states.

A liquid crystal display device in this specification refers to an imagedisplay device, a display device, or a light source (including alighting device). Further, the liquid crystal display device alsoincludes any of the following modules in its category: a module to whicha connector such as a flexible printed circuit (FPC), a tape automatedbonding (TAB) tape, or a tape carrier package (TCP) is attached; amodule having a TAB tape or a TCP at the end of which a printed wiringboard is provided; and a module in which an integrated circuit (IC) isdirectly mounted on a display element by a chip-on-glass (COG) method.

Next, the appearance and a cross section of a liquid crystal displaypanel, which is one embodiment of the liquid crystal display device ofthe present invention, will be described with reference to FIGS. 24A1,24A2, and 24B. FIGS. 24A1 and 24A2 are top views of panels in which aliquid crystal element 4013 is sealed with a sealant 4005 between afirst substrate 4001 and a second substrate 4006. FIG. 24B is across-sectional view taken along line M-N of FIGS. 24A1 and 24A2.

In FIGS. 24A1, 24A2, and 24B, the sealant 4005 is provided so as tosurround a pixel portion 4002 and a scan line driver circuit 4004 whichare provided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Therefore, the pixel portion 4002 and the scan line driver circuit4004 are sealed together with a liquid crystal layer 4008, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. Thereis no particular limitation on the liquid crystal layer 4008 in thisembodiment, but a liquid crystal material exhibiting a blue phase isused. A liquid crystal material exhibiting a blue phase has a shortresponse time of one millisecond or less from the state of applying novoltage to the state of applying voltage, whereby short-time response ispossible. The liquid crystal material exhibiting a blue phase includes aliquid crystal and a chiral agent. The chiral agent is employed to alignthe liquid crystal in a helical structure and to make the liquid crystalexhibit a blue phase. For example, a liquid crystal material into whicha chiral agent is mixed at 5 wt % or more may be used for the liquidcrystal layer. As the liquid crystal, a thermotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like is used.

In FIG. 24A1, a signal line driver circuit 4003 that is formed using asingle crystal semiconductor film or a polycrystalline semiconductorfilm over a substrate separately prepared is mounted in a region that isdifferent from the region surrounded by the sealant 4005 over the firstsubstrate 4001.

Note that FIG. 24A2 illustrates an example in which part of the signalline driver circuit is formed over the first substrate 4001. A signalline driver circuit 4003 b is formed over the first substrate 4001, anda signal line driver circuit 4003 a formed using a single crystalsemiconductor film or a polycrystalline semiconductor film is mountedover a separately-prepared substrate.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 24A1 illustrates anexample in which the signal line driver circuit is mounted by a COGmethod, and FIG. 24A2 illustrates an example in which the signal linedriver circuit is mounted by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001 include a plurality oftransistors. FIG. 24B illustrates the transistor 4010 included in thepixel portion 4002 and the transistor 4011 included in the scan linedriver circuit 4004. Over the transistors 4010 and 4011, a firstprotective insulating layer 4020 and a resin layer 4021 which is asecond protective insulating layer, and a third protective insulatinglayer 4022 are provided. The transistors which are described in any oneof Embodiments 1 to 4 can be used as the transistors 4010 and 4011. Inthis embodiment, the transistors 4010 and 4011 are n-channel transistorseach including an oxide semiconductor layer for a channel formationregion.

The transistors 4010 and 4011 are covered with the first protectiveinsulating layer 4020, the resin layer 4021 which is the secondprotective insulating layer, and the third protective insulating layer4022. The first protective insulating layer 4020 is provided over and incontact with the oxide semiconductor layers of the transistors 4010 and4011 and a gate insulating layer 4019.

The resin layer 4021 which is the second protective insulating layer andserves as a planarizing insulating film can be formed from an organicmaterial having heat resistance, such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that the resin layer4021 may be formed by stacking a plurality of insulating films formed ofthese materials. The resin layer 4021 is a light-transmitting resinlayer and a photosensitive polyimide resin is used in this embodiment.

There is no particular limitation on the formation method of theinsulating layer, and the following method can be employed depending onthe material: a method such as a sputtering method, an SOG method, spincoating, dip coating, spray coating, or a droplet discharging method(e.g., ink jetting, screen printing, or offset printing), or with a tool(equipment) such as a doctor knife, a roll coater, a curtain coater, ora knife coater.

Note that the third protective insulating layer 4022 is provided toprevent entry of an impurity element (such as sodium) which floats inair, such as an organic substance, a metal substance, or water vapor,and which contaminates the oxide semiconductor layer, and the thirdprotective insulating layer 4022 is preferably a dense film. Theprotective film may be formed using either a single layer or a stackedlayer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film,an aluminum nitride film, aluminum oxynitride film, and/or an aluminumnitride oxide film by a PCVD method or a sputtering method.

Further, the third protective insulating layer 4022 is formed using asilicon nitride film obtained under a low power condition by a plasmaCVD method. Further, a base insulating layer 4007 which is formed usinga silicon nitride film and the third protective insulating layer 4022are in contact with each other outside the pixel portion to surround theresin layer 4021 which is the second protective insulating layer. Thus,the transistors 4010 and 4011 are encapsulated with silicon nitridefilms, whereby the reliability of the transistors 4010 and 4011 isimproved.

A second gate electrode 4028 is formed over the first protectiveinsulating layer 4020 and in a position overlapping with the oxidesemiconductor layer of the transistor 4011. A second gate electrode 4029is formed over the third protective insulating layer 4022 and in aposition overlapping with the oxide semiconductor layer of thetransistor 4010.

A pixel electrode layer 4030 and a common electrode layer 4031 areprovided over the first substrate 4001, and the pixel electrode layer4030 is electrically connected to the transistor 4010. The second gateelectrodes 4028 and 4029 can have the same potential as the commonelectrode layer 4031. The second gate electrodes 4028 and 4029 areformed in the same step as the common electrode layer 4031. Further, ifthe second gate electrodes 4028 and 4029 are formed using alight-blocking material, they can also serve as light-blocking layersshielding the oxide semiconductor layers of the transistors 4011 and4010 from light.

Alternatively, the second gate electrodes 4028 and 4029 can have adifferent potential from the common electrode layer 4031. In this case,a control line electrically connected to the second gate electrodes 4028and 4029 is provided and the threshold voltage of each of thetransistors 4011 and 4010 is controlled with a potential of the controlline.

Note that the structures of the transistors are not limited to the abovedescription, and the second gate electrodes 4028 and 4029 may beconnected to the first gate electrode, or they may be in a floatingstate.

The liquid crystal element 4013 includes the pixel electrode layer 4030,the common electrode layer 4031, and the liquid crystal layer 4008. Inthis embodiment, a method is used in which grayscale is controlled bygenerating an electric field which is substantially parallel to asubstrate (i.e., in a lateral direction) to move liquid crystalmolecules in a plane parallel to the substrate. In such a method, anelectrode structure used in an in plane switching (IPS) mode or a fringefield switching (FFS) mode can be used. Note that a polarizing plate4032 and a polarizing plate 4033 are provided on the outer sides of thefirst substrate 4001 and the second substrate 4006, respectively.

For the first substrate 4001 and the second substrate 4006, a glasssubstrate, a plastic substrate, or the like having a light-transmittingproperty can be used. As the plastic substrate, a fiberglass-reinforcedplastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film,or an acrylic resin film can be used. Moreover, a sheet in whichaluminum foil is sandwiched between PVF films or polyester films canalso be used.

A post spacer 4035 is obtained by selective etching of an insulatingfilm and is provided in order to control the thickness (a cell gap) ofthe liquid crystal layer 4008. Note that the shape of the spacer is notlimited thereto, and a spherical spacer may alternatively be used. Thecolumnar post spacer 4035 is located to overlap with the second gateelectrode 4029.

FIGS. 24A1, 24A2, and 24B illustrate examples of liquid crystal displaydevices in which a polarizing plate is provided on the outer side (theview side) of a substrate; however, the polarizing plate may be providedon the inner side of the substrate.

Furthermore, a light-blocking layer serving as a black matrix may beprovided to an appropriate position. In FIGS. 24A1, 24A2, and 24B, alight-blocking layer 4034 is provided on the second substrate 4006 sideso as to cover the transistors 4010 and 4011. By provision of thelight-blocking layer 4034, contrast can be further improved and thetransistor can operate stably.

When the light-blocking layer 4034 is provided, the intensity ofincident light on the oxide semiconductor layers of the transistors canbe attenuated; accordingly, electric characteristics of the transistorscan be prevented from being varied due to photosensitivity of the oxidesemiconductor layers and the transistors can operate stably.

The pixel electrode layer 4030, the common electrode layer 4031, and thesecond gate electrodes 4028 and 4029 can be formed from alight-transmitting conductive material such as indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide (hereinafter referred to as ITO), indium zincoxide, or indium tin oxide to which silicon oxide is added.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can also be used for the pixelelectrode layer 4030, the common electrode layer 4031, and the secondgate electrodes 4028 and 4029.

Note that a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, and the scan linedriver circuit 4004 or the pixel portion 4002 from an FPC 4018.

Further, since the transistor is easily broken by static electricity andthe like, a protective circuit for protecting the driver circuits ispreferably provided over the same substrate for a gate line or a sourceline. The protective circuit is preferably formed using a nonlinearelement in which an oxide semiconductor is used.

In FIGS. 24A1, 24A2, and 24B, a connection terminal electrode 4015 andthe pixel electrode layer 4030 are formed from the same layer, and aterminal electrode 4016 and source and drain electrode layers of thetransistors 4010 and 4011 are formed from the same layer.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4017.

FIGS. 24A1, 24A2, and 24B illustrate an example in which the signal linedriver circuit 4003 is separately formed and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be formed separately andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be formed separately and then mounted.

FIG. 25 illustrates an example of a cross-sectional structure of aliquid crystal display device in which an element substrate 2600 and acounter substrate 2601 are attached to each other with a sealant 2602,and an element layer 2603 including a transistor or the like and aliquid crystal layer 2604 are provided between the substrates.

In the case where color display is performed, for example,light-emitting diodes which emit light of plural colors may be arrangedin a backlight portion. In the case of an RGB mode, a red light-emittingdiode 2610R, a green light-emitting diode 2610G, and a bluelight-emitting diode 2610B are disposed in each of the regions intowhich a display area of the liquid crystal display device is divided.

A polarizing plate 2606 is provided on the outer side of the countersubstrate 2601, and a polarizing plate 2607 and an optical sheet 2613are provided on the outer side of the element substrate 2600. A lightsource is formed using the red light-emitting diode 2610R, the greenlight-emitting diode 2610G, the blue light-emitting diode 2610B, and areflective plate 2611. An LED control circuit 2614 provided for acircuit substrate 2612 is connected to a wiring circuit portion 2608 ofthe element substrate 2600 via a flexible wiring board 2609 and furtherincludes an external circuit such as a control circuit or a power sourcecircuit.

In this embodiment, an example in which LEDs are individually made toemit light by this LED control circuit 2614, so that a field-sequentialliquid crystal display device is formed; however, an embodiment of thepresent invention is not limited thereto. A cold cathode tube or a whiteLED may be used as a light source of backlight, and a color filter maybe provided.

Further, in this embodiment, an example of an electrode structure usedin the IPS mode is described; however, there is no particularlylimitation on the electrode structure mode. The following mode can beused: a TN (twisted nematic) mode, an MVA (multi-domain verticalalignment) mode, a PVA (patterned vertical alignment) mode, an ASM(axially symmetric aligned micro-cell) mode, an OCB (optical compensatedbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(antiferroelectric liquid crystal) mode, or the like.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 9

In this embodiment, an example of an electronic paper will be describedas a semiconductor device which includes a plurality of transistorsincluding an oxide semiconductor layer.

FIG. 26A is a cross-sectional structure of an active matrix electronicpaper. As a transistor 581 used in a display portion of thesemiconductor device, the transistor which is described in any one ofEmbodiments 1 to 4 can be employed.

The electronic paper of FIG. 26A is an example of a display device inwhich a twisting ball display system is employed. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are used for a display element and arearranged between a first electrode layer and a second electrode layer,and a potential difference is generated between the first electrodelayer and the second electrode layer to control orientation of thespherical particles, so that display is performed.

The transistor 581 has a bottom-gate structure. A first electrode layer587 is electrically connected to a source or drain electrode through anopening formed in a first protective insulating layer 584, a resin layer585 which is a second protective insulating layer, and a thirdprotective insulating layer 586. The first protective insulating layer584 covers the transistor 581. A second gate electrode 582 is providedbelow and in contact with the resin layer 585 which is provided over thefirst protective insulating layer 584, and the third protectiveinsulating layer 586 is provided to cover the second gate electrode 582.An oxide semiconductor layer of the transistor 581 is protected by thefirst protective insulating layer 584, the resin layer 585 which is thesecond protective insulating layer, the second gate electrode 582, andthe third protective insulating layer 586.

Between the first electrode layer 587 and a second electrode layer 588,spherical particles 589 each having a black region 590 a, a white region590 b, and a cavity 594 are provided. A space around the sphericalparticles 589 is filled with a filler 595 such as a resin (see FIG.26A). The first electrode layer 587 corresponds to the pixel electrodeand the second electrode layer 588 corresponds to the common electrode.The second electrode layer 588 is electrically connected to a commonpotential line provided over the same substrate as the transistor 581.With the use of a common connection portion, the second electrode layer588 can be electrically connected to the common potential line throughconductive particles provided between a pair of substrates.

Further, instead of the twisting ball, an electrophoretic element canalso be used. A microcapsule having a diameter of about 10 μm to 200 μmin which transparent liquid, positively charged white microparticles,and negatively charged black microparticles are encapsulated, is used.In the microcapsule which is provided between the first electrode layerand the second electrode layer, when a potential difference is generatedbetween the first electrode layer and the second electrode layer, thewhite microparticles and the black microparticles move to oppositesides, so that white or black can be displayed. A display element usingthis principle is an electrophoretic display element and is called anelectronic paper in general. The electrophoretic display element hashigher reflectance than a liquid crystal display element, and thus, anauxiliary light is unnecessary, power consumption is low, and a displayportion can be recognized in a dim place. In addition, even when poweris not supplied to the display portion, an image which has beendisplayed once can be maintained. Accordingly, in the case where theelectric paper has a structure in which a signal and electric power arewirelessly supplied from an electric wave source, a displayed image canbe held even if a semiconductor device having a display function isdistanced from the electric wave source.

By using the transistor manufactured by the process described in any oneof Embodiments 1 to 4 as a switching element, an electronic paper can bemanufactured as a semiconductor device at low cost. An electronic papercan be used for electronic devices of a variety of fields as long asthey can display data. For example, an electronic paper can be appliedto an electronic book (e-book) reader, a poster, an advertisement in avehicle such as a train, displays of various cards such as a creditcard, and the like. Examples of such electronic devices are illustratedin FIG. 26B.

FIG. 26B illustrates an example of an electronic book reader 2700. Theelectronic book reader 2700 includes two housings, a first housing 2701and a second housing 2703. The first housing 2701 and the second housing2703 are combined with a hinge 2711 so that the electronic book reader2700 can be opened and closed with the hinge 2711 as an axis. With sucha structure, the electronic book reader 2700 can be operated like apaper book.

A first display portion 2705 and a second display portion 2707 areincorporated in the first housing 2701 and the second housing 2703,respectively. The first display portion 2705 and the second displayportion 2707 may be configured to display one image or different images.In the case where the first display portion 2705 and the second displayportion 2707 display different images, for example, a display portion onthe right side (the first display portion 2705 in FIG. 26B) can displaytext and a display portion on the left side (the second display portion2707 in FIG. 26B) can display graphics.

In the electronic book reader 2700 in FIG. 26B, the first housing 2701is provided with an operation portion and the like. For example, thefirst housing 2701 is provided with a power switch 2721, an operationkey 2723, a speaker 2725, and the like. With the operation key 2723,pages can be turned. Note that a keyboard, a pointing device, or thelike may be provided on the surface of the housing, on which the displayportion is provided. Further, an external connection terminal (anearphone terminal, a USB terminal, a terminal that can be connected tovarious cables such as an AC adapter and a USB cable, or the like), arecording medium insert portion, or the like may be provided on the backsurface or the side surface of the housing. Further, the electronic bookreader 2700 may have a function of an electronic dictionary.

The electronic book reader 2700 may be configured to transmit andreceive data by wireless communication. The structure can be employed inwhich desired book data or the like is purchased and downloaded from anelectronic book server by wireless communication.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 10

A semiconductor device using the transistor described in any one ofEmbodiments 1 to 4 can be applied to a variety of electronic devices(including an amusement machine). Examples of electronic devices includea television set (also referred to as a television or a televisionreceiver), a monitor of a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, amobile phone handset (also referred to as a mobile phone or a mobilephone device), a portable game console, a portable information terminal,an audio reproducing device, a large-sized game machine such as apachinko machine, and the like.

In the television set in FIG. 27A, a display portion 9603 isincorporated in a housing 9601. The display portion 9603 can display animage. Here, the rear side of the housing 9601 is supported so that thetelevision set is fixed to a wall 9600.

The television set illustrated in FIG. 27A can be operated with anoperation switch of the housing 9601 or a remote controller 9610.Channels and volume can be controlled with an operation key 9609 of theremote controller 9610 so that an image displayed on the display portion9603 can be controlled. Further, the remote controller 9610 may beprovided with a display portion 9607 for displaying data output from theremote controller 9610.

Note that the television set illustrated in FIG. 27A is provided with areceiver, a modem, and the like. With the use of the receiver, generaltelevision broadcasting can be received. Moreover, when the televisionset is connected to a communication network by wired or wirelessconnection via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers) datacommunication can be performed.

FIG. 27B is a portable game machine and includes two housings, a housing9881 and a housing 9891, which are connected with a joint portion 9893so that the portable game machine can be opened or folded. A displayportion 9882 and a display portion 9883 are incorporated in the housing9881 and the housing 9891, respectively. In addition, the portable gamemachine illustrated in FIG. 27B is provided with a speaker portion 9884,a recording medium insert portion 9886, an LED lamp 9890, input means(operation keys 9885, a connection terminal 9887, a sensor 9888 (havinga function of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, odor, or infrared ray), and amicrophone 9889), and the like. Needless to say, the structure of theportable game machine is not limited to that described above. Theportable game machine may have a structure in which additional accessoryequipment is provided as appropriate as long as at least a semiconductordevice according to an example of the present invention is provided. Theportable game machine illustrated in FIG. 27B has a function of readinga program or data stored in a recording medium to display it on thedisplay portion, and a function of sharing information with anotherportable game machine by wireless communication. Note that a function ofthe portable game machine illustrated in FIG. 27B is not limited tothose described above, and the portable game machine can have a varietyof functions.

FIG. 28A illustrates an example of a mobile phone handset 1000. Themobile phone handset 1000 is provided with a display portion 1002incorporated in a housing 1001, operation buttons 1003, an externalconnection port 1004, a speaker 1005, a microphone 1006, and the like.

When the display portion 1002 of the mobile phone handset 1000illustrated in FIG. 28A is touched with a finger or the like, data canbe input into the mobile phone handset 1000. Further, operation such asmaking calls and texting can be performed by touching the displayportion 1002 with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode which is a combination of the twomodes, that is, a combination of the display mode and the input mode.

For example, in the case of making a call or texting, a text input modemainly for inputting text is selected for the display portion 1002 sothat text displayed on a screen can be inputted. In that case, it ispreferable to display a keyboard or number buttons on almost all area ofthe screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone handset 1000, display on the screen of the display portion1002 can be automatically changed by determining the orientation of themobile phone handset 1000 (whether the mobile phone handset 1000 isplaced horizontally or vertically for a landscape mode or a portraitmode).

The screen modes are changed by touching the display portion 1002 orusing the operation buttons 1003 of the housing 1001. Alternatively, thescreen modes may be changed depending on the kind of the image displayedon the display portion 1002. For example, when a signal of an imagedisplayed on the display portion 1002 is the one of moving image data,the screen mode is changed to the display mode. When the signal is theone of text data, the screen mode is changed to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 1002 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 1002 is touched with a palm or a finger, wherebypersonal identification can be performed. Further, by providing abacklight or a sensing light source which emits near-infrared light inthe display portion 1002, an image of a finger vein, a palm vein, or thelike can be taken.

The cellular phone in FIG. 28B has a display device 9410 in a housing9411, which includes a display portion 9412 and operation buttons 9413,and a communication device 9400 in a housing 9401, which includesoperation buttons 9402, an external input terminal 9403, a microphone9404, a speaker 9405, and a light-emitting portion 9406 that emits lightwhen a phone call is received. The display device 9410 which has adisplay function can be detached from or attached to the communicationdevice 9400 which has a phone function by moving in directionsrepresented by arrows. Thus, the display device 9410 and thecommunication device 9400 can be attached to each other along theirshort sides or long sides. In addition, when only the display functionis needed, the display device 9410 can be detached from thecommunication device 9400 and used alone. Images, input information, orthe like can be transmitted or received by wireless or wirecommunication between the communication device 9400 and the displaydevice 9410, each of which has a rechargeable battery.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Example 1

One of methods for examining reliability of transistors is abias-temperature stress test (hereinafter, referred to as a BT test).The BT test is one kind of accelerated test and can evaluate change incharacteristics, caused by long-term usage, of transistors in a shorttime. In particular, the amount of shift in threshold voltage of thetransistor between before and after the BT test is an importantindicator for examining reliability. Between before and after the BTtest, the small amount of shift in threshold voltage means highreliability.

Specifically, the temperature of a substrate over which a transistor isformed (substrate temperature) is set at fixed temperature, a source anda drain of the transistor are set at the same potential, and a gate issupplied with potential different from those of the source and the drainfor a certain period. The substrate temperature may be set asappropriate in accordance with the purpose of the test. A test in thecase where potential applied to the gate is higher than potentials ofthe source and the drain is referred to as a +BT test, and a test in thecase where potential applied to the gate is lower than potentials of thesource and the drain is referred to as a −BT test.

The stress conditions for the BT test can be determined by setting thesubstrate temperature, electric field intensity applied to a gateinsulating film, or a time period of application of electric field. Theelectric field intensity applied to a gate insulating film can bedetermined by dividing the potential difference between the gatepotential and the source and drain potential by the thickness of thegate insulating film. For example, in the case where the electric fieldintensity applied to the 100-nm-thick gate insulating film is to be setto 2 MV/cm, the potential difference may be set to 20 V.

In this example, results of a BT test performed on three kinds ofsamples are described. The samples are subjected to heat treatment undera nitrogen atmosphere at 250° C., 350° C., and 450° C., which isperformed before formation of a source and a drain in manufacture of atransistor.

Note that “voltage” generally indicates a difference between potentialsof two points, and “potential” indicates a static electric energy(electrical potential energy) unit charge which is at a point in astatic electric field has. However, in an electronic circuit, adifference between a potential at a certain point and a referencepotential (e.g., a ground potential) is often referred to as thepotential at a certain point. Thus, in the following description, when adifference between a potential at a certain point and a referencepotential (e.g., a ground potential) is referred to as the potential ata certain point, the potential at a certain point means the voltageexcept for the case where definition is particularly given.

As the BT test, a +BT test and a −BT test were performed under suchconditions that a substrate temperature was 150° C., an electric fieldintensity applied to a gate insulating film was 2 MV/cm, and a timeperiod for application was one hour.

First, the +BT test is described. In order to measure initialcharacteristics of a transistor subjected to the BT test, a change incharacteristics of the source-drain current (hereinafter, referred to asthe drain current) was measured, under the conditions where thesubstrate temperature was set to 40° C., the voltage between a sourceand a drain (hereinafter, the drain voltage) was set to 10 V, and thevoltage between a source and a gate (hereinafter, the gate voltage) waschanged in the range of −20 V to +20 V. That is, Vg-Id characteristicswere measured. Here, as a countermeasure against moisture-absorptiononto surfaces of the samples, the substrate temperature was set to 40°C. However, the measurement may be performed at room temperature (25°C.) or lower if there is no particular problem.

Next, the substrate temperature was increased to 150° C., and then, thepotentials of the source and the drain of the transistor were set to 0V. After that, the voltage was applied to the gate so that the electricfield intensity applied to the gate insulating film was 2 MV/cm. In thiscase, the thickness of the gate insulating film of the transistor was100 nm. The gate was supplied with +20 V of voltage, and the gatesupplied with the voltage was kept for one hour. Note that although thetime period for voltage application was one hour here, the time periodmay be changed as appropriate in accordance with the purpose.

Next, the substrate temperature was lowered to 40° C. while the voltagewas kept on being applied to the source, the drain, and the gate. Ifapplication of the voltage is stopped before the substrate temperaturewas completely lowered to 40° C., the transistor which has been damagedduring the BT test is repaired by the influence of residual heat. Thus,lowering of the substrate temperature needs to be performed withapplication of the voltage. After the substrate temperature was loweredto 40° C., application of the voltage was terminated.

Then, the Vg-Id characteristics were measured under the conditions sameas those for the measurement of the initial characteristics, so that theVg-Id characteristics after the +BT test were obtained.

Next, the −BT test is described. The −BT test was performed with theprocedure similar to the +BT test, but has a different point from the+BT test, in that the voltage applied to the gate after the substratetemperature is increased to 150° C. is set to −20 V.

In the BT test, it is important to use a transistor which has been neversubjected to a BT test. For example, if a −BT test is performed with useof a transistor which has been once subjected to a +BT test, the resultsof the −BT test cannot be evaluated correctly due to influence of the+BT test which has been performed previously. Similarly, if thetransistor which has been once subjected to a +BT test is used foranother +BT test, the results cannot be evaluated correctly. However,the usage of the transistor is not limited to the above in the casewhere the BT test is performed repeatedly in consideration of suchinfluence.

FIGS. 29A to 29C show the Vg-Id characteristics of the transistorsbefore and after the +BT tests. FIG. 29A shows the +BT test results oftransistors each formed in such a manner that heat treatment isperformed under a nitrogen atmosphere at 250° C. before formation of asource and a drain. FIG. 29B shows the +BT test results of transistorseach formed in such a manner that heat treatment is performed under anitrogen atmosphere at 350° C. before formation of a source and a drain.FIG. 29C shows the +BT test results of transistors each formed in such amanner that heat treatment is performed under a nitrogen atmosphere at450° C. before formation of a source and a drain.

FIGS. 30A to 30C show the Vg-Id characteristics of the transistorsbefore and after the −BT tests. FIG. 30A shows the −BT test results oftransistors each formed in such a manner that heat treatment isperformed under a nitrogen atmosphere at 250° C. before formation of asource and a drain. FIG. 30B shows the −BT test results of transistorseach formed in such a manner that heat treatment is performed under anitrogen atmosphere at 350° C. before formation of a source and a drain.FIG. 30C shows the −BT test results of transistors each formed in such amanner that heat treatment is performed under a nitrogen atmosphere at450° C. before formation of a source and a drain.

Note that in FIGS. 29A to 29C and FIGS. 30A to 30C, the second gateelectrode has a three-layer structure in which a titanium layer (50 nm),an aluminum layer (100 nm), and a titanium layer (5 nm) are stacked. Thesecond gate electrode of each pixel is led individually. Note that as acomparative example, results of +BT tests of when the second gateelectrode is not provided are shown in FIGS. 31A to 31C, and results of−BT tests of when the second gate electrode is not provided are shown inFIGS. 32A to 32C. FIG. 31A shows the +BT test results at 250° C., FIG.31B shows the +BT test results at 350° C., and FIG. 31C shows the +BTtest results at 450° C. FIG. 32A shows the −BT test results at 250° C.,FIG. 32B shows the −BT test results at 350° C., and FIG. 32C shows the−BT test results at 450° C.

Note that in each of FIGS. 31A to 31C and FIGS. 32A to 32C, thehorizontal axis shows the gate voltage (V_(g)) and the vertical axisshows the drain current (I_(d)), and both of them are represented in alogarithm scale. In each of FIGS. 31A to 31C and FIGS. 32A to 32C, asolid line represents initial characteristics and a dotted linerepresents characteristics after the stress is applied.

In terms of the amount of shift in the threshold voltage after the +BTtest, it is found from FIGS. 29A to 29C and FIGS. 31A to 31C that theshift amount at 350° C. is smaller than that at 250° C. and that theshift amount at 450° C. is smaller than that at 350° C. That is, thehigher the temperature of heat treatment, the smaller the amount ofshift in the threshold voltage after +BT tests becomes. In addition, itis found from the comparison between FIGS. 30A to 30C and FIGS. 32A to32C that the amount of shift in the threshold voltage after the −BT testbecomes small by provision of the second gate electrode.

As can be seen from FIGS. 29A to 29C and FIGS. 31A to 31C, in the casewhere the temperature of the heat treatment performed before formationof the source and drain is about 400° C. or higher, the reliability inat least the +BT test can be improved. As can be seen from thecomparison between FIGS. 30A to 30C and FIGS. 32A to 32C, in the casewhere the second gate electrode is provided, the reliability in the −BTtest can be improved. Therefore, in the case where the temperature ofthe heat treatment performed before formation of the source and drain isabout 400° C. or higher and the second gate electrode is provided, thereliability in the +BT test and the −BT test can be improved.

As described in this example, according to one embodiment of the presentinvention, the reliability in both of the +BT test and the −BT test canbe improved.

Note that the transistor having high reliability in the −BT test asdescribed above is particularly useful for application to a drivercircuit in a driver circuit portion of a display device.

Example 2

In this example, heat treatment was performed on a plurality of samplesunder a nitrogen atmosphere at heat temperatures whose conditions weredetermined Such a plurality of samples were measured with thermaldesorption spectroscopy (hereinafter referred to as TDS). Measurementresults are shown in FIG. 34, FIG. 35, and FIG. 36.

The TDS is used for detecting and identifying a gas component dischargedor generated from the samples by a quadrupole mass analyzer; thus, a gasand a molecule discharged from surfaces and insides of the samples canbe observed. Discharge or generation of gas from the samples occurswhile the samples are heated and the temperature is rising in highvacuum. With use of a TDS (product name: 1024 amu QMS) manufactured byESCO Ltd., under a condition where the rising temperature was atapproximately 10° C./min, measurement was performed. At the beginning ofthe measurement, the pressure was 1×10⁻⁸ (Pa), and during themeasurement, the pressure was at a degree of vacuum of about 1×10⁻⁷(Pa).

FIG. 34 is a graph showing TDS measurement results of comparison betweena sample (comparative sample) which includes only a glass substrate anda sample (Sample 1) where an In—Ga—Zn—O-based non-single-crystal filmwith an original thickness of 50 nm (an actual thickness obtained afteretching is about 30 nm) is formed over a glass substrate. FIG. 34 showsTDS measurement results obtained by measuring H₂O. Discharge ofimpurities such as moisture (H₂O) from the In—Ga—Zn—O-basednon-single-crystal film can be confirmed from a peak in the vicinity of300° C.

FIG. 35 is a graph showing comparison of samples, which shows TDSmeasurement results of H₂O. The comparison was performed on thefollowing samples: the sample (Sample 1) where an In—Ga—Zn—O-basednon-single-crystal film with an original thickness of 50 nm is formedover a glass substrate; a sample (Sample 2) where the structure ofSample 1 is subjected to heat treatment for an hour at 350° C. under anair atmosphere; and a sample (Sample 3) where the structure of Sample 1is subjected to heat treatment for an hour at 350° C. under a nitrogenatmosphere. From the results shown in FIG. 35, a peak in the vicinity of300° C. of Sample 3 is lower than that of Sample 2. Thus, discharge ofmoisture (H₂O) due to heat treatment performed under a nitrogenatmosphere can be confirmed. Moreover, it is found that heat treatmentperformed under a nitrogen atmosphere reduces impurities such asmoisture (H₂O) more than heat treatment performed under an airatmosphere.

FIG. 36 is a graph showing comparison of samples, which shows TDSmeasurement results of H₂O. The comparison was performed on thefollowing samples: the sample (Sample 1) where an In—Ga—Zn—O-basednon-single-crystal film with an original thickness of 50 nm is formedover a glass substrate; a sample (Sample 4) where the structure ofSample 1 is subjected to heat treatment for an hour at 250° C. under anitrogen atmosphere; the sample (Sample 3) where the structure of Sample1 is subjected to heat treatment for an hour at 350° C. under a nitrogenatmosphere; a sample (Sample 5) where the structure of Sample 1 issubjected to heat treatment for an hour at 450° C. under a nitrogenatmosphere; and a sample (Sample 6) where the structure of Sample 1 issubjected to heat treatment for 10 hours at 350° C. under a nitrogenatmosphere. From the results shown in FIG. 36, it is found that thehigher the heat temperature within the measurement temperature rangeunder a nitrogen atmosphere is, the smaller the amount of impuritiessuch as moisture (H₂O) discharged from the In—Ga—Zn—O-basednon-single-crystal film becomes.

In addition, from the graphs of FIG. 35 and FIG. 36, two peaks can beconfirmed: a first peak in the vicinity of 200° C. to 250° C., whichindicates discharge of impurities such as moisture (H₂O); and a secondpeak in the vicinity of 300° C., which indicates discharge of impuritiessuch as moisture (H₂O).

Note that even in the case where the sample which has been subjected toheat treatment at 450° C. under a nitrogen atmosphere is left at roomtemperature in an air atmosphere approximately for one week, dischargeof moisture at 200° C. or higher was not observed. Thus, it is foundthat by performing heat treatment, the In—Ga—Zn—O-basednon-single-crystal film becomes stable.

Further, FIG. 33 shows measurement results of carrier concentrations.Conditions of heat temperature under a nitrogen atmosphere were set to150° C., 175° C., 200° C., 225° C., 250° C., 275° C., 300° C., 325° C.,350° C., 375° C., 400° C., 425° C., and 450° C., and a carrierconcentration at each temperature was measured. When an oxide insulatingfilm is formed over the In—Ga—Zn—O-based non-single-crystal film, acarrier concentration of 1×10¹⁴/cm³ or lower, which is indicated by adotted line in FIG. 33, was obtained.

Next, measurements of the carrier concentration and Hall mobility aredescribed. FIG. 37A illustrates a three-dimensional view of aproperty-evaluation sample 510 for evaluating properties (the carrierconcentrations and Hall mobility) of an oxide semiconductor film (anIn—Ga—Zn—O-based non-single-crystal film). Here, the property-evaluationsample 510 was fabricated and subjected to Hall effect measurement atroom temperature. The carrier concentration and Hall mobility of theoxide semiconductor film were evaluated. The property-evaluation sample510 was fabricated in the following manner: an insulating film 501including silicon oxynitride was formed over a substrate 500, an oxidesemiconductor film 502 with a size of 10 mm×10 mm, which serves as anevaluation object, was formed over the insulating film 501, andelectrodes 503, 504, 505, and 506 each having a diameter of 1 mm wereformed over the oxide semiconductor film 502. FIG. 37B shows themeasurement result of the Hall mobility, and FIG. 37C shows themeasurement result of the conductivity. The carrier concentrations ofthe oxide semiconductor film obtained by the Hall effect measurement areshown in FIG. 33.

From the results of FIG. 33, FIG. 34, FIG. 35, and FIG. 36, it is foundthat there is a relation, at 250° C. or higher, between discharge ofimpurities such as moisture (H₂O) from the In—Ga—Zn—O-basednon-single-crystal film and change in carrier concentration. That is,when the impurities such as moisture (H₂O) are discharged from theIn—Ga—Zn—O-based non-single-crystal film, the carrier concentration isincreased.

Moreover, H, O, OH, H₂, O₂, N, N₂, and Ar, in addition to H₂O, were eachmeasured by TDS. The measurement resulted in that peaks of H, O, and OHwere observed clearly but peaks of H₂, O₂, N, N₂, and Ar were notobserved. As samples of the above measurement, a structure where anIn—Ga—Zn—O-based non-single-crystal film with an original thickness of50 nm was formed over a glass substrate was used. The conditions of heattreatment were set as follows: heat treatment under a nitrogenatmosphere at 250° C. for an hour; that under a nitrogen atmosphere at350° C. for an hour; that under a nitrogen atmosphere at 350° C. for tenhours; and that under a nitrogen atmosphere at 450° C. for an hour. Ascomparative samples, a structure in which heat treatment was notperformed on an In—Ga—Zn—O-based non-single-crystal film and a structureincluding only a glass substrate were measured. FIG. 38, FIG. 39, FIG.40, and FIG. 41 show TDS results of H, O, OH, and H₂, respectively. Notethat under the above conditions of heat treatment, the oxygen densityunder a nitrogen atmosphere is 20 ppm or lower.

Example 3

With respect to an oxide semiconductor layer including a region havinghigh oxygen density and a region having low oxygen density, a phenomenonin which oxygen is diffused in accordance with heat treatment wassimulated. The result thereof will be described with reference to FIG.42 and FIG. 43 in this example. As software for the simulation,Materials Explorer 5.0 manufactured by Fujitsu Limited was used.

FIG. 42 illustrates a model of an oxide semiconductor layer which wasused for the simulation. Here, a structure in which a layer 705 havinghigh oxygen density were stacked over a layer 703 having low oxygendensity was employed for an oxide semiconductor layer 701.

The layer 703 having low oxygen density was formed to have an amorphousstructure including In atoms, Ga atoms, Zn atoms, and O atoms, where thenumbers of In atoms, Ga atoms, and Zn atoms were each 15 and the numberof O atoms was 54.

In addition, the layer 705 having high oxygen density was formed to havean amorphous structure including In atoms, Ga atoms, Zn atoms, and Oatoms, where the numbers of In atoms, Ga atoms, and Zn atoms were each15 and the number of O atoms was 66.

The density of the oxide semiconductor layer 701 was set to 5.9 g/cm³.

Next, the classical molecular dynamics (MD) simulation was performed onthe oxide semiconductor layer 701 under conditions of NVT ensemble and atemperature of 250° C. The time step was set to 0.2 fs, and the totalsimulation time was set to 200 ps. In addition, Born-Mayer-Hugginspotential was used for the potentials of metal-oxygen bonding andoxygen-oxygen bonding. Moreover, movement of atoms at an upper endportion and a lower end portion of the oxide semiconductor layer 701 wasfixed.

The simulation results are shown in FIG. 43. In z-axis coordinates, therange of 0 nm to 1.15 nm indicates the layer 703 having low oxygendensity, and the range of 1.15 nm to 2.3 nm indicates the layer 705having high oxygen density. The distribution of oxygen densities beforethe MD simulation is indicated by a solid line 707, and the distributionof oxygen densities after the MD simulation is indicated by a dashedline 709.

The solid line 707 shows that the oxide semiconductor layer 701 has highoxygen densities in a region ranging from an interface between the layer703 having low oxygen density and the layer 705 having high oxygendensity to the layer 705 having high oxygen density. On the other hand,the dashed line 709 shows that the oxygen density is uniform in thelayer 703 having low oxygen density and the layer 705 having high oxygendensity.

From the above, when there is non-uniformity in the distribution ofoxygen concentration as in the stack of the layer 703 having low oxygendensity and the layer 705 having high oxygen density, it is found thatthe oxygen diffuses from where the oxygen density is higher to where theoxygen density is lower by heat treatment and thus the oxygen densitybecomes uniform.

That is, as described in Embodiment 1, since the oxygen density at theinterface between the oxide semiconductor layer 403 and the first oxideinsulating layer 407 is increased by formation of the first protectiveinsulating layer 407 over the oxide semiconductor layer 403 with use ofan insulating oxide, the oxygen diffuses to the oxide semiconductorlayer 403 where the oxygen density is low and thus the oxidesemiconductor layer 403 has higher resistance. As described above, thereliability of a transistor included in a display device which is oneembodiment of the present invention can be improved.

This application is based on Japanese Patent Application serial no.2009-159052 filed with Japan Patent Office on Jul. 3, 2009, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a first conductivelayer; a first insulating layer over the first conductive layer; anoxide semiconductor layer over the first insulating layer; a secondinsulating layer over the oxide semiconductor layer; a second conductivelayer and a third conductive layer over the second insulating layer; athird insulating layer over the second conductive layer and the thirdconductive layer; a fourth conductive layer over the third insulatinglayer; a fourth insulating layer over the fourth conductive layer; and apixel electrode electrically connected to the second conductive layer,wherein each of the fourth conductive layer and the pixel electrodecomprises a light-transmitting conductive material, wherein the fourthconductive layer extends beyond both side edges of the oxidesemiconductor layer in a channel width direction, and wherein the fourthconductive layer overlaps a light-blocking layer.
 3. The semiconductordevice according to claim 2, wherein the fourth conductive layer and thepixel electrode are formed from a same material.
 4. The semiconductordevice according to claim 2, wherein the oxide semiconductor layercomprises indium, gallium, and zinc.
 5. The semiconductor deviceaccording to claim 2, wherein the oxide semiconductor layer comprises amicrocrystalline region.
 6. The semiconductor device according to claim2, wherein the oxide semiconductor layer comprises an oxygen-excessregion.
 7. The semiconductor device according to claim 2, wherein thelight-blocking layer is the first conductive layer.
 8. The semiconductordevice according to claim 2, wherein the light-blocking layer is a partof a black matrix.
 9. The semiconductor device according to claim 2,wherein the first conductive layer and the fourth conductive layer aresupplied with a same potential.
 10. The semiconductor device accordingto claim 2, wherein the first conductive layer and the fourth conductivelayer are supplied with a different potential.
 11. A semiconductordevice comprising: a first conductive layer; a first insulating layerover the first conductive layer; an oxide semiconductor layer over thefirst insulating layer; a second insulating layer over the oxidesemiconductor layer; a second conductive layer and a third conductivelayer over the second insulating layer; a third insulating layer overthe second conductive layer and the third conductive layer; a fourthconductive layer over the third insulating layer; a fourth insulatinglayer over the fourth conductive layer; and a pixel electrodeelectrically connected to the second conductive layer, wherein each ofthe fourth conductive layer and the pixel electrode comprises alight-transmitting conductive material, wherein the fourth conductivelayer extends beyond both side edges of the oxide semiconductor layer ina channel width direction, wherein the fourth conductive layer overlapsa light-blocking layer, wherein the first conductive layer iselectrically connected to a first line, and wherein the fourthconductive layer is electrically connected to a second line
 12. Thesemiconductor device according to claim 11, wherein the fourthconductive layer and the pixel electrode are formed from a samematerial.
 13. The semiconductor device according to claim 11, whereinthe oxide semiconductor layer comprises indium, gallium, and zinc. 14.The semiconductor device according to claim 11, wherein the oxidesemiconductor layer comprises a microcrystalline region.
 15. Thesemiconductor device according to claim 11, wherein the oxidesemiconductor layer comprises an oxygen-excess region.
 16. Thesemiconductor device according to claim 11, wherein the light-blockinglayer is the first conductive layer.
 17. The semiconductor deviceaccording to claim 11, wherein the light-blocking layer is a part of ablack matrix.
 18. The semiconductor device according to claim 11,wherein the first line and the second line are electrically connected toa same driver circuit.
 19. The semiconductor device according to claim11, wherein the first line is electrically connected to a first drivercircuit, and wherein the second line is electrically connected to asecond driver circuit.